docs: add missing verb (#15765)

Author: mj0nez

docs/source/cheat_sheet_py3.rst

@@ -104,7 +104,7 @@ Functions
        print(value + "!" * excitement)
 
    # Note that arguments without a type are dynamically typed (treated as Any)
-   # and that functions without any annotations not checked
+   # and that functions without any annotations are not checked
    def untyped(x):
        x.anything() + 1 + "string"  # no errors