minor #4419 [DependencyInjection] Add missing space in code (michaelperrin)

This PR was merged into the 2.3 branch.

Discussion
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[DependencyInjection] Add missing space in code

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | N/A

That's really a small contribution, but it solves a slight PSR issue.

Commits
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aff9c2c Add missing space in code

Author: wouterj

components/dependency_injection/tags.rst

@@ -211,7 +211,7 @@ To begin with, change the ``TransportChain`` class::
         public function getTransport($alias)
         {
             if (array_key_exists($alias, $this->transports)) {
-               return $this->transports[$alias];
+                return $this->transports[$alias];
             }
         }
     }