doc: fix formatting on platform-hooks

Signed-off-by: Thomas Ingleby <thomas.c.ingleby@intel.com>

docs/platform-hooks.md

@@ -3,17 +3,17 @@ This feature of MRAA should only be used by developers defining the board defini
 
 ##Types of Hooks
 
-** REPLACE **
+###REPLACE
 Defining a replace function will entirely replace the associate function. This should only be done if your new function can handle everything the mraa function would normally.
 
-** PRE **
+###PRE
 Any functionality defined here will be performed when the main function is called.
 
-** POST **
+###POST
 Any functionality perfomed here is done just before the normal function returns. All post functions will have passed into them the return value that would normally be returned.
 
 ##Hooks
-** gpio **
+### GPIO
  * init (pre-post) - On RAW functions
  * mode (replace-pre-post)
  * dir (replace-pre-post)