minor symfony#9951 Updated associations.rst (ansien12)

This PR was submitted for the master branch but it was merged into the 4.0 branch instead (closes symfony#9951).

Discussion
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Updated associations.rst

Updated to reflect correct nullable status of $category.

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Commits
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d42fe11 Updated associations.rst

Author: javiereguiluz

doctrine/associations.rst

@@ -155,7 +155,7 @@ the ``Product`` entity (and getter & setter methods):
                 return $this->category;
             }
 
-            public function setCategory(?Category $category): self
+            public function setCategory(Category $category): self
             {
                 $this->category = $category;