minor #7079 Update cache.rst (andreybolonin, javiereguiluz)

This PR was submitted for the 3.1 branch but it was merged into the 3.2 branch instead (closes #7079).

Discussion
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Update cache.rst

Commits
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e4033a4 Removed the example about Redis
1f370ca Update cache.rst

Author: xabbuh

components/cache.rst

@@ -37,7 +37,7 @@ meaning of some key concepts:
 **Adapter**
     It implements the actual caching mechanism to store the information in the
     filesystem, in a database, etc. The component provides several ready to use
-    adapters for common caching backends (Redis, APCu, etc.)
+    adapters for common caching backends (Redis, APCu, Doctrine, PDO, etc.)
 
 Basic Usage
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