users_guide: Fix "CancelSynchronousIo" casing

Reviewers: bgamari, angerman

Reviewed By: angerman

Subscribers: angerman, rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D4200

docs/users_guide/ffi-chap.rst

@@ -160,7 +160,7 @@ Unix systems
 
 Windows systems
     [Vista and later only] The RTS calls the Win32 function
-    ``CancelSynchronousIO``, which will cause a blocking I/O operation
+    ``CancelSynchronousIo``, which will cause a blocking I/O operation
     to return with the error ``ERROR_OPERATION_ABORTED``.
 
 If the system call is successfully interrupted, it will return to