minor symfony#9952 Update conventions.rst (poliveras)

This PR was submitted for the 4.0 branch but it was merged into the 2.8 branch instead (closes symfony#9952).

Discussion
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Update conventions.rst

Fix plural in sentence

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Commits
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aaa1f48 Update conventions.rst

Author: javiereguiluz

contributing/code/conventions.rst

@@ -26,7 +26,7 @@ When an object has a "main" many relation with related "things"
 * ``count()``
 * ``keys()``
 
-The usage of these methods are only allowed when it is clear that there
+The usage of these methods is only allowed when it is clear that there
 is a main relation:
 
 * a ``CookieJar`` has many ``Cookie`` objects;