tests: stack: fix improper assert message

I find that there has a description error and a spelling
mistake in assert message. So I fixed this.

Signed-off-by: Lixin Guo <lixinx.guo@intel.com>

tests/kernel/stack/stack/src/test_stack_contexts.c

@@ -183,7 +183,7 @@ void test_stack_alloc_thread2thread(void)
 	/** Requested buffer allocation from the test pool.*/
 	ret = k_stack_alloc_init(&kstack_test_alloc, (STACK_SIZE/2)+1);
 	zassert_true(ret == -ENOMEM,
-			"resource pool is smaller then requested buffer");
+			"requested buffer is smaller than resource pool");
 }
 
 static void low_prio_wait_for_stack(void *p1, void *p2, void *p3)