minor #6654 Update upload_file.rst (liubinas)

This PR was submitted for the 3.1 branch but it was merged into the 2.7 branch instead (closes #6654).

Discussion
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Update upload_file.rst

seperate -> separate

Commits
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b4bed39 Update upload_file.rst

Author: xabbuh

cookbook/controller/upload_file.rst

@@ -222,7 +222,7 @@ Creating an Uploader Service
 ----------------------------
 
 To avoid logic in controllers, making them big, you can extract the upload
-logic to a seperate service::
+logic to a separate service::
 
     // src/AppBundle/FileUploader.php
     namespace AppBundle;