malloc_info.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/malloc_info.3

@@ -226,7 +226,7 @@ main(int argc, char *argv[])
     blockSize = atoi(argv[3]);
     sleepTime = (argc > 4) ? atoi(argv[4]) : 0;
 
-    thr = calloc(numThreads, sizeof(pthread_t));
+    thr = calloc(numThreads, sizeof(*thr));
     if (thr == NULL)
         errExit("calloc");