minor symfony#12878 Update doctrine.rst (johnnypeck)

This PR was submitted for the 5.0 branch but it was merged into the 4.3 branch instead (closes symfony#12878).

Discussion
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Update doctrine.rst

Grammar.

Commits
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c23db39 Update doctrine.rst

Author: javiereguiluz

doctrine.rst

@@ -742,7 +742,7 @@ Querying with the Query Builder
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 Doctrine also provides a `Query Builder`_, an object-oriented way to write
-queries. It is recommended to use this when queries and build dynamically (i.e.
+queries. It is recommended to use this when queries are built dynamically (i.e.
 based on PHP conditions)::
 
     // src/Repository/ProductRepository.php