Minor doc fix [ci skip]

Author: mpeterv

doc/index.html

@@ -817,7 +817,7 @@ <h3>
 <h3>
 <a name="generating-usage-and-help-messages" class="anchor" href="#generating-usage-and-help-messages"><span class="octicon octicon-link"></span></a>Generating usage and help messages</h3>
 
-<p><code>:get_help()</code> and <code>get_usage:()</code> methods of Parser and Command classes can be used to generate their help and usage messages. </p>
+<p><code>:get_help()</code> and <code>:get_usage()</code> methods of Parser and Command classes can be used to generate their help and usage messages. </p>
 
 <h3>
 <a name="parsing-algorithm" class="anchor" href="#parsing-algorithm"><span class="octicon octicon-link"></span></a>Parsing algorithm</h3>