minor #1283 fix typo (nicoder)

This PR was submitted for the master branch but it was merged into the 1.3 branch instead (closes #1283).

Discussion
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fix typo

Commits
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6d0944dd fix typo

doc/usage.rst

@@ -459,7 +459,7 @@ the defaults for new controllers.
 Error Handlers
 --------------
 
-When an exception is thrown, error handlers allows you to display a custom
+When an exception is thrown, error handlers allow you to display a custom
 error page to the user. They can also be used to do additional things, such as
 logging.