docs: fix esp_pthread example syntax error

docs/en/api-reference/system/esp_pthread.rst

@@ -13,11 +13,15 @@ This module offers Espressif specific extensions to the pthread library that can
 
 Example to tune the stack size of the pthread:
 
-.. highlight:: c
+.. code-block:: c
 
-::
+    void * thread_func(void * p)
+    {
+        printf("In thread_func\n");
+        return NULL;
+    }
 
-   main()
+    void app_main(void)
     {
         pthread_t t1;
 
@@ -30,14 +34,14 @@ Example to tune the stack size of the pthread:
 
 The API can also be used for inheriting the settings across threads. For example:
 
-.. highlight:: c
-
-::
+.. code-block:: c
        
     void * my_thread2(void * p)
     {   
         /* This thread will inherit the stack size of 4K */
         printf("In my_thread2\n");
+
+        return NULL;
     }
 
     void * my_thread1(void * p)
@@ -45,11 +49,12 @@ The API can also be used for inheriting the settings across threads. For example
         printf("In my_thread1\n");
         pthread_t t2;
         pthread_create(&t2, NULL, my_thread2);
+
+        return NULL;
     }
 
-   main()
+    void app_main(void)
     {
-
         pthread_t t1;
 
         esp_pthread_cfg_t cfg = esp_create_default_pthread_config();