minor #13520 [twig] Fix getComputedData in new twig profiler (jeremy-derusse)

This PR was merged into the 2.7 branch.

Discussion
----------

[twig] Fix getComputedData in new twig profiler

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | should be
| Fixed tickets | NA
| License       | MIT
| Doc PR        | NA

Add a small fix to #13428

Commits
-------

e8ca06a Fix getComputedData

src/Symfony/Bridge/Twig/DataCollector/TwigDataCollector.php

@@ -93,7 +93,7 @@ class TwigDataCollector extends DataCollector implements LateDataCollectorInterf
             $this->computed = $this->computeData($this->getProfile());
         }
 
-        return $this->computed['index'];
+        return $this->computed[$index];
     }
 
     private function computeData(\Twig_Profiler_Profile $profile)