Typo: isis -> is (#495)

Fixes inspect.isgeneratorfunction().

stdlib/3/inspect.pyi

@@ -23,7 +23,7 @@ def ismodule(object: object) -> bool: ...
 def isclass(object: object) -> bool: ...
 def ismethod(object: object) -> bool: ...
 def isfunction(object: object) -> bool: ...
-def isisgeneratorfunction(object: object) -> bool: ...
+def isgeneratorfunction(object: object) -> bool: ...
 def isgenerator(object: object) -> bool: ...
 
 # Python 3.5+