docs: fixed typo in spelling of first (#4428)

Author: kajananchinniah

include/pybind11/detail/common.h

@@ -436,7 +436,7 @@ PYBIND11_WARNING_POP
 
 /** \rst
     This macro creates the entry point that will be invoked when the Python interpreter
-    imports an extension module. The module name is given as the fist argument and it
+    imports an extension module. The module name is given as the first argument and it
     should not be in quotes. The second macro argument defines a variable of type
     `py::module_` which can be used to initialize the module.