Update rust package license metadata link

As noted by @wking at https://github.com/github/choosealicense.com/pull/563/files#r160834170

Author: mlinksva

_includes/sidebar.html

@@ -14,7 +14,7 @@ <h3>How to apply this license</h3>
     {% endif %}
     {% assign xgpl = false %}
     {% if page.spdx-id contains 'GPL' %}{% assign xgpl = true %}{% endif %}
-    <p class="note"><strong>Optional: </strong> Add <strong><code>{{ page.spdx-id }}</code>{% if xgpl %}+{% endif %}</strong>{% if xgpl %} (or <strong><code>{{ page.spdx-id }}</code></strong> to disallow future versions){% endif %} to your project's package description, if applicable (e.g., <a href="https://docs.npmjs.com/files/package.json#license">Node.js</a>, <a href="http://guides.rubygems.org/specification-reference/#license=">Ruby</a>, and <a href="http://doc.crates.io/manifest.html#package-metadata">Rust</a>). This will ensure the license is displayed in package directories.</p>
+    <p class="note"><strong>Optional: </strong> Add <strong><code>{{ page.spdx-id }}</code>{% if xgpl %}+{% endif %}</strong>{% if xgpl %} (or <strong><code>{{ page.spdx-id }}</code></strong> to disallow future versions){% endif %} to your project's package description, if applicable (e.g., <a href="https://docs.npmjs.com/files/package.json#license">Node.js</a>, <a href="http://guides.rubygems.org/specification-reference/#license=">Ruby</a>, and <a href="https://doc.rust-lang.org/cargo/reference/manifest.html#package-metadata">Rust</a>). This will ensure the license is displayed in package directories.</p>
   </div>
 
   {% if page.source %}