docs: fix audits sub-table array name

[anglepoised]

Fixes the first example in the README, which has an incorrect sub-table array name - [plugins.inputs] instead of [plugins.inputs.audits] - that can lead to issues processing netlify.toml e.g. any header definitions in the file won't get parsed.

Anyone copypasta-ing this first example (like, er, me) would likely end up with issues in their config.

The other examples in README look good; just the first that has issues.

Using the version from the README with no changes:

[[plugins]]
  package = "@netlify/plugin-lighthouse"

  # optional, fails build when a category is below a threshold
  [plugins.inputs.thresholds]
    performance = 0.9
    accessibility = 0.9
    best-practices = 0.9
    seo = 0.9
    pwa = 0.9

  # optional, deploy the lighthouse report to a path under your site
  [plugins.inputs]
    output_path = "reports/lighthouse.html"

...gives a log that look something like this:

8:32:16 PM: Starting post processing
8:32:16 PM: Incorrect TOML configuration format: Key inputs is already used as table key
8:32:16 PM: Post processing - HTML
8:32:16 PM: Incorrect TOML configuration format: Key inputs is already used as table key
8:32:16 PM: Incorrect TOML configuration format: Key inputs is already used as table key
8:32:16 PM: Post processing - header rules
8:32:16 PM: Incorrect TOML configuration format: Key inputs is already used as table key
8:32:17 PM: Post processing - redirect rules
8:32:17 PM: Incorrect TOML configuration format: Key inputs is already used as table key
8:32:17 PM: Post processing done
8:32:17 PM: Section completed: postprocessing
8:32:17 PM: Site is live ✨

Adding a valid [[headers]] section, like this:

[[headers]]
  for = "/*"
  [headers.values]
    Referrer-Policy = "strict-origin-when-cross-origin"

[[plugins]]
  package = "@netlify/plugin-lighthouse"

  # optional, fails build when a category is below a threshold
  [plugins.inputs.thresholds]
    performance = 0.9
    accessibility = 0.9
    best-practices = 0.9
    seo = 0.9
    pwa = 0.9

  # optional, deploy the lighthouse report to a path under your site
  [plugins.inputs]
    output_path = "reports/lighthouse.html"

...gives an identical log to the above:

8:32:58 PM: Starting post processing
8:32:58 PM: Incorrect TOML configuration format: Key inputs is already used as table key
8:32:58 PM: Post processing - HTML
8:32:58 PM: Incorrect TOML configuration format: Key inputs is already used as table key
8:32:59 PM: Incorrect TOML configuration format: Key inputs is already used as table key
8:32:59 PM: Post processing - header rules
8:32:59 PM: Incorrect TOML configuration format: Key inputs is already used as table key
8:32:59 PM: Post processing - redirect rules
8:32:59 PM: Incorrect TOML configuration format: Key inputs is already used as table key
8:32:59 PM: Post processing done
8:32:59 PM: Section completed: postprocessing
8:33:00 PM: Site is live ✨

...but also fails to parse the headers, saying "No header rules processed" in the Deploy summary.

[aitchiss]

Thank you so much for spotting and fixing this!

[netlify]

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