Improve description of TrimKeyFixer (#1022)

Author: kubawerlos

README.md

@@ -713,7 +713,7 @@ A class that implements the `__toString()` method must explicitly implement the
 ```
 
 #### TrimKeyFixer
-A string array and yield keys must be trimmed and have no double spaces.
+The string key of an array or generator must be trimmed and have no double spaces.
 ```diff
  <?php
  $array = [

src/Fixer/TrimKeyFixer.php

@@ -23,7 +23,7 @@ final class TrimKeyFixer extends AbstractFixer
     public function getDefinition(): FixerDefinitionInterface
     {
         return new FixerDefinition(
-            'A string array and yield keys must be trimmed and have no double spaces.',
+            'The string key of an array or generator must be trimmed and have no double spaces.',
             [new CodeSample(<<<'PHP'
                 <?php
                 $array = [