minor #4788 Controller is a callable (timglabisch)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #4788).

Discussion
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Controller is a callable

Hello,

i am not sure about this PR but the statement ``A controller is a PHP function`` is wrong.
A controller can be everything that is callable by php.

Commits
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3e98b3a Controller is a callable

book/controller.rst

@@ -4,7 +4,7 @@
 Controller
 ==========
 
-A controller is a PHP function you create that takes information from the
+A controller is a PHP callable you create that takes information from the
 HTTP request and constructs and returns an HTTP response (as a Symfony
 ``Response`` object). The response could be an HTML page, an XML document,
 a serialized JSON array, an image, a redirect, a 404 error or anything else