Use `()` for `get_function_sig_hook` docs (#15499)

docs/source/extending_mypy.rst

diff --git a/docs/source/extending_mypy.rst b/docs/source/extending_mypy.rst
index daf8636..506f548 100644
--- a/docs/source/extending_mypy.rst
+++ b/docs/source/extending_mypy.rst
@@ -159,7 +159,7 @@
 This is a good choice if the return type is too complex
 to be expressed by regular python typing.
 
-**get_function_signature_hook** is used to adjust the signature of a function.
+**get_function_signature_hook()** is used to adjust the signature of a function.
 
 **get_method_hook()** is the same as ``get_function_hook()`` but for methods
 instead of module level functions.