doc: use <code> for a few file paths that didn't

Spotted while seeing CL 187818 in master.

Change-Id: Ic3f42558f6833fc35580b2b2a15f2c5aba5713c0
Reviewed-on: https://go-review.googlesource.com/c/go/+/187822Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/contribute.html

@@ -238,15 +238,16 @@ $ git codereview help
 </pre>
 
 <p>
-prints help text, not an error. If it prints an error, make sure that $GOPATH/bin
-is in your $PATH.
+prints help text, not an error. If it prints an error, make sure that
+<code>$GOPATH/bin</code> is in your <code>$PATH</code>.
 </p>
 
 <p>
 On Windows, when using git-bash you must make sure that
 <code>git-codereview.exe</code> is in your <code>git</code> exec-path.
 Run <code>git --exec-path</code> to discover the right location then create a
-symbolic link or just copy the executable from $GOPATH/bin to this directory.
+symbolic link or just copy the executable from <code>$GOPATH/bin</code> to this
+directory.
 </p>