pthread_create.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/pthread_create.3

@@ -361,7 +361,7 @@ main(int argc, char *argv[])
 
     /* Allocate memory for pthread_create() arguments */
 
-    tinfo = calloc(num_threads, sizeof(struct thread_info));
+    tinfo = calloc(num_threads, sizeof(*tinfo));
     if (tinfo == NULL)
         handle_error("calloc");