minor #4375 Removed the redundant usage of layer. (micheal)

This PR was submitted for the 2.5 branch but it was merged into the 2.3 branch instead (closes #4375).

Discussion
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Removed the redundant usage of layer.

Since DBAL is database abstraction layer, and is defined in the document as such, to me it reads funny to have it as database abstraction layer layer. It sounds nitpicky, I know, but I find it an easier read this way. Also, the possessive DBAL stuck out ("DBAL's layer") as the DBAL doesn't own a layer, it is the layer whereas Doctrine does own the layer and is properly defined in the header.

Commits
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27339c7 One more again. I could have sworn I had them lined up.
84164fa Removed the possession from Doctrine.
693698e Shorten the = to match the length of the text.
5ea2902 Removed the redundant usage of layer.

cookbook/doctrine/dbal.rst

@@ -1,12 +1,12 @@
 .. index::
    pair: Doctrine; DBAL
 
-How to Use Doctrine's DBAL Layer
-================================
+How to Use Doctrine DBAL
+========================
 
 .. note::
 
-    This article is about Doctrine DBAL's layer. Typically, you'll work with
+    This article is about the Doctrine DBAL. Typically, you'll work with
     the higher level Doctrine ORM layer, which simply uses the DBAL behind
     the scenes to actually communicate with the database. To read more about
     the Doctrine ORM, see ":doc:`/book/doctrine`".