docs: correct the function prototype defined in the stub file in simplest_c example (#937)

In the simplest_c example, the stub file defines the function `square`
to take two arguments. However, it only takes one in the C-extension
implementation. This PR corrects the stub file to make it consistent
with the C-extension implementation.

Author: LinZhihao-723

tests/packages/simplest_c/src/simplest/_module.pyi

@@ -1 +1 @@
-def square(__x: float, __y: float) -> float: ...
+def square(__x: float) -> float: ...