Update changelog.md

Author: microbuilder

changelog.md

@@ -45,12 +45,12 @@
 - Added separate thread for callbacks to allow most API functions to be
   invoked directly inside the callback handler
 
-#### Tools
+### Tools
 
 - Updated `tools/nrfutil-0.5.2/nordicsemi/dfu/init_packet.py` to avoid a build
   error on OS X and Linux.
 
-#### New Examples
+### New Examples
 
 - Hardware/
     - hwpwm