fix(docs): eliminate 2 types of sphinx warnings: (#6928)

Co-authored-by: Liam <30486941+liamHowatt@users.noreply.github.com>

docs/porting/os.rst

@@ -10,7 +10,7 @@ However, in the following conditions it's valid to call LVGL related
 functions:
 
 - In *events*. Learn more in :ref:`events`.
-- In *lv_timer*. Learn more in :ref:`timer`.
+- In *lv_timer*. Learn more in :ref:`timers`.
 
 Tasks and threads
 -----------------

docs/porting/timer_handler.rst

@@ -1,4 +1,4 @@
-.. _timer:
+.. _timer_handler:
 
 =============
 Timer Handler
@@ -48,11 +48,11 @@ In an OS environment, you can use it together with the **delay** or
 .. code:: c
 
    while (1) {
-      uint32_t time_till_next = lv_timer_handler(); 
+      uint32_t time_till_next = lv_timer_handler();
       os_delay_ms(time_till_next); /* delay to avoid unnecessary polling */
    }
 
-To learn more about timers visit the :ref:`timer`
+To learn more about timers visit the :ref:`timers`
 section.
 
 API