minor symfony#11198 Update unit_testing.rst (favianioel)

This PR was submitted for the 4.2 branch but it was merged into the 3.4 branch instead (closes symfony#11198).

Discussion
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Update unit_testing.rst

While following the tutorial changing the property from $controllerResolver to $resolver generated a bug because in previous example from httpkernel in the file the property was called like this
$controller = $this->controllerResolver->getController($request);
It is not a fix but it could help the people that follow the tutorial not get stuck 👍

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Commits
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3853c5b Update unit_testing.rst

Author: xabbuh

create_framework/unit_testing.rst

@@ -57,13 +57,13 @@ resolver. Modify the framework to make use of them::
     class Framework
     {
         protected $matcher;
-        protected $resolver;
+        protected $controllerResolver;
         protected $argumentResolver;
 
         public function __construct(UrlMatcherInterface $matcher, ControllerResolverInterface $resolver, ArgumentResolverInterface $argumentResolver)
         {
             $this->matcher = $matcher;
-            $this->resolver = $resolver;
+            $this->controllerResolver = $resolver;
             $this->argumentResolver = $argumentResolver;
         }