fix(rustup): use <pre><code> not just <code>

`<code>` is an inline element, and was forcing overflow on the page on
small screens. Wrapping it in a `<pre>` element properly lets it fill
its space but then overflow into a scroll region as necessary.

Author: chriskrycho

templates/tools/rustup.hbs

@@ -1,15 +1,15 @@
 <div class="row">
   <div id="platform-instructions-unix" class="instructions" style="display: block;">
     <p>It looks like you’re running macOS, Linux, or another Unix-like OS. To download Rustup and install Rust, run the following in your terminal, then follow the on-screen instructions.</p>
-    <code>curl https://sh.rustup.rs -sSf | sh</code>
+    <pre><code>curl https://sh.rustup.rs -sSf | sh</code></pre>
   </div>
   <div id="platform-instructions-win" class="instructions" style="display: none;">
     <p>It looks like you’re running Windows. To install Rust, download and run</p>
     <a href="https://win.rustup.rs" class="button button-secondary">rustup‑init.exe</a>
     <p>...then follow the onscreen instructions.</p>
     <p><b>Windows Subsystem for Linux</b></p>
     <p>If you’re a Windows Subsystem for Linux user run the following in your terminal, then follow the on-screen instructions to install Rust.</p>
-    <code>curl https://sh.rustup.rs -sSf | sh</code>
+    <pre><code>curl https://sh.rustup.rs -sSf | sh</code></pre>
   </div>
   <div id="platform-instructions-unknown" class="instructions" style="display: none;">
     <!-- unrecognized platform: ask for help -->