Use ::before pseudo-element for the shell prompt

Of the `git --version` code snippet.

Author: ugultopu

app/assets/stylesheets/reference.scss

@@ -146,11 +146,11 @@ h3.plumbing {
 
     code {
       user-select: all;
-    }
 
-    span.prompt {
-      user-select: none;
-      font-weight: bold;
+      &::before {
+        content: '$ ';
+        font-weight: bold;
+      }
     }
   }
 }

app/views/doc/_versions.html.erb

@@ -35,6 +35,6 @@
   </ol>
   <footer>
     <p>Check your version of git by running</p>
-    <code><span class="prompt">$ </span>git --version</code>
+    <code>git --version</code>
   </footer>
 </div>