minor #3243 Syntax error in doc (JakeFr)

This PR was merged into the 1.x branch.

Discussion
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Syntax error in doc

This code sample in advanced.rst is not highlighted in php because the second semicolon is missing

Commits
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fbfac78 Syntax error in doc

doc/advanced.rst

@@ -660,7 +660,7 @@ An extension is a class that implements the following interface::
 
 To keep your extension class clean and lean, inherit from the built-in
 ``\Twig\Extension\AbstractExtension`` class instead of implementing the interface as it provides
-empty implementations for all methods:
+empty implementations for all methods::
 
     class Project_Twig_Extension extends \Twig\Extension\AbstractExtension
     {