mbstowcs.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/mbstowcs.3

@@ -186,7 +186,7 @@ main(int argc, char *argv[])
     /* Allocate wide character string of the desired size.  Add 1
        to allow for terminating null wide character (L\(aq\e0\(aq). */
 
-    wcs = calloc(mbslen + 1, sizeof(wchar_t));
+    wcs = calloc(mbslen + 1, sizeof(*wcs));
     if (wcs == NULL) {
         perror("calloc");
         exit(EXIT_FAILURE);