running_mypy.rst: Fix typo (#14004)

Introduced in #14003

Author: AlexWaygood

docs/source/running_mypy.rst

@@ -334,7 +334,7 @@ to your mypy command to install all known missing stubs:
 
     mypy --install-types
 
-This is slower than explicitly installing stubs, since if effectively
+This is slower than explicitly installing stubs, since it effectively
 runs mypy twice -- the first time to find the missing stubs, and
 the second time to type check your code properly after mypy has
 installed the stubs. It also can make controlling stub versions harder,