bpo-29918: Add missed "const" modifiers in C API documentation. (#846)

GitHub-Issue-Link: python/cpython#74104

Author: serhiy-storchaka

Doc/howto/clinic.rst

@@ -1050,7 +1050,7 @@ Currently Argument Clinic supports only a few return converters:
     DecodeFSDefault
 
 None of these take parameters.  For the first three, return -1 to indicate
-error.  For ``DecodeFSDefault``, the return type is ``char *``; return a NULL
+error.  For ``DecodeFSDefault``, the return type is ``const char *``; return a NULL
 pointer to indicate an error.
 
 (There's also an experimental ``NoneType`` converter, which lets you