docs(toolbar): set proper role value

[alan-agius4]

Based on https://www.w3.org/TR/wai-aria/#role_definitions there is no such role as header, but rather it should heading

[ngbot]

I see that you just added the pr: merge ready label, but the following checks are still failing:
     missing required label: "target: *"
     status "ci/circleci: build" is pending
     status "continuous-integration/travis-ci/pr" is pending

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