Improve formatting in BAT.md (llvm#170254)

Make "Header" a subheading to improve readability in the Functions table
section.

Author: Akshiitaa06

bolt/docs/BAT.md

@@ -61,6 +61,7 @@ Functions table:
 
 ### Functions table
 Hot and cold functions tables share the encoding except differences marked below.
+
 Header:
 | Entry  | Encoding | Description |
 | ------ | ----- | ----------- |