fix: correct typo in dart reserved word list and capitalize nav link titles

Fix missing space between 'false' and 'for' in the reserved words example
on the syntax documentation page. Also capitalize 'Method calls' to
'Method Calls' and 'Error handling' to 'Error Handling' in the template
navigation sidebar for consistent title casing.

doc/site/template.html

@@ -23,7 +23,7 @@
     <h2>language</h2>
     <ul>
       <li><a href="syntax.html">Syntax</a></li>
-      <li><a href="method-calls.html">Method calls</a></li>
+      <li><a href="method-calls.html">Method Calls</a></li>
       <li><a href="variables.html">Variables</a></li>
       <li><a href="branching.html">Branching</a></li>
       <li><a href="looping.html">Looping</a></li>
@@ -40,7 +40,7 @@
     <h2>usage</h2>
     <ul>
       <li>Concurrency</li>
-      <li>Error handling</li>
+      <li>Error Handling</li>
       <li>Standalone</li>
       <li>Embedding</li>
     </ul>