Fix indent level for build.rs example (#1431)

* Better fix code blocks in `/about`

This is a followup to #1315 to fix the extra blank line at the top and
to fix the indentation.

Co-authored-by: Camelid <camelidcamel@gmail.com>

Author: jyn514

templates/core/Cargo.toml.example

@@ -1,5 +1,4 @@
-{# The example Cargo.toml used in about.html #}
-[package]
+{# The example Cargo.toml used in about.html #}[package]
 name = "test"
 
 [package.metadata.docs.rs]

templates/core/about/builds.html

@@ -31,7 +31,7 @@ <h4 id="setting-a-readme"> <a href="#setting-a-readme">Setting a README</a> </h4
     <h4 id="detecting-docsrs"> <a href="#detecting-docsrs">Detecting Docs.rs</a> </h4>
     <p>
         To recognize Docs.rs from <code>build.rs</code> files, you can test for the environment variable <code>DOCS_RS</code>, e.g.:
-        {% filter dedent(levels=4) -%}
+        {% filter dedent(levels=3) -%}
         <pre><code class="lang-rust">if let Ok(_) = std::env::var("DOCS_RS") {
                 // ... your code here ...
             }</code></pre>