doc: fix minor formatting issues in contribute.html

- remove <code> from <pre>
- replace `` with <code></code>

Change-Id: I46f0aec8b7645e2ac8cb53bca73aed55441acd65
Reviewed-on: https://go-review.googlesource.com/42612
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/contribute.html

@@ -305,10 +305,10 @@ Go to a directory where you want the source to appear and run the following
 command in a terminal.
 </p>
 
-<pre><code>
+<pre>
 $ git clone https://go.googlesource.com/go
 $ cd go
-</code></pre>
+</pre>
 
 <h3 id="master">Contributing to the main Go tree</h3>
 
@@ -396,8 +396,9 @@ and
 
 <p>
 Once you have the changes queued up, you will want to commit them.
-In the Go contribution workflow this is done with a `git change` command,
-which creates a local branch and commits the changes directly to that local branch.
+In the Go contribution workflow this is done with a <code>git</code>
+<code>change</code> command, which creates a local branch and commits the changes
+directly to that local branch.
 </p>
 
 <pre>
@@ -418,9 +419,9 @@ then <code>git</code> <code>commit</code>.)
 </p>
 
 <p>
-As the `git commit` is the final step, Git will open an editor to ask for a
-commit message.
-(It uses the editor named by the <code>$EDITOR</code> environment variable,
+As the <code>git</code> <code>commit</code> is the final step, Git will open an
+editor to ask for a commit message. (It uses the editor named by
+the <code>$EDITOR</code> environment variable,
 <code>vi</code> by default.)
 
 The file will look like: