patch 9.1.0649: Wrong comment for "len" argument of call_simple_func()

Problem:  Wrong comment for "len" argument of call_simple_func().
Solution: Remove the "or -1 to use strlen()".  Also change its type to
          size_t to remove one cast. (zeertzjq)

closes: #15410

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/userfunc.c

@@ -4058,7 +4058,7 @@ theend:
     int
 call_simple_func(
     char_u	*funcname,	// name of the function
-    int		len,		// length of "name" or -1 to use strlen()
+    size_t	len,		// length of "name"
     typval_T	*rettv)		// return value goes here
 {
     int		ret = FAIL;