README: Fix feature sub-section levels

Each main feature should have its own sub-sections
for settings, limitations, etc.
This was not the case for the combat borders :
lower the title level for its subsections

README.md

@@ -31,12 +31,12 @@ highlighting the tokens that have played for this combat turn.
 The module supports going back and forth in the combat rounds, as well as going back in the turn order.
 (Though because of limitations of the Fabula Ultima system, does not allow going *forward* in the turn order.)
 
-## Settings
+### Settings
 
 - An image to use for the default/idle border
 - An image to use for the took turn/played/inactive border
 
-## Limitations
+### Limitations
 
 There are currently two main issues that need to be fixed :
 1. The tokens will not be updated when the GM is not on the scene.