minor twigphp#2598 Update advanced.rst (pfwd)

This PR was submitted for the 2.x branch but it was merged into the 1.x branch instead (closes twigphp#2598).

Discussion
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Update advanced.rst

I noticed that the rot13Provider wasn't called correctly as it was trying to access a local variable instead of calling $this

Commits
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b42fd64 Update advanced.rst

doc/advanced.rst

@@ -800,7 +800,7 @@ The simplest way to use methods is to define them on the extension itself::
 
         public function rot13($value)
         {
-            return $rot13Provider->rot13($value);
+            return $this->rot13Provider->rot13($value);
         }
     }
 
@@ -849,7 +849,7 @@ It is now possible to move the runtime logic to a new
 
         public function rot13($value)
         {
-            return $rot13Provider->rot13($value);
+            return $this->rot13Provider->rot13($value);
         }
     }