getgrouplist.3: Use sizeof consistently

Use ``sizeof`` consistently through all the examples in the following
way:

- Use the name of the variable instead of its type as argument for
  ``sizeof``.

	Rationale:
	https://www.kernel.org/doc/html/v5.8/process/coding-style.html#allocating-memory

Signed-off-by: Alejandro Colomar <colomar.6.4.3@gmail.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man3/getgrouplist.3

@@ -164,7 +164,7 @@ main(int argc, char *argv[])
 
     ngroups = atoi(argv[2]);
 
-    groups = malloc(sizeof(gid_t) * ngroups);
+    groups = malloc(sizeof(*groups) * ngroups);
     if (groups == NULL) {
         perror("malloc");
         exit(EXIT_FAILURE);