minor #6227 Update testing.rst (dvapelnik)

This PR was merged into the 2.3 branch.

Discussion
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Update testing.rst

Q | A
----|----
Doc fix? | yes (small fix with link to document)
New docs? | no
Applies to | 2.3
Fixed tickets |

Commits
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fed16fe Update testing.rst

book/testing.rst

@@ -857,7 +857,7 @@ section:
 Learn more
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-* The :doc:`chapter about tests in the Symfony Framework Best Practices </best_practices/tests>`
+* :doc:`The chapter about tests in the Symfony Framework Best Practices </best_practices/tests>`
 * :doc:`/components/dom_crawler`
 * :doc:`/components/css_selector`
 * :doc:`/cookbook/testing/http_authentication`