Update README.md

Co-authored-by: per1234 <[email protected]>

Author: facchinm

README.md

@@ -61,7 +61,7 @@ PROFILE=release ./mbed-os-to-arduino -a NANO_RP2040_CONNECT:NANO_RP2040_CONNECT
 
 The `PROFILE` environment variable tunes the compilation profiles (defaults to `DEVELOP`). Other available profiles are `DEBUG` and `RELEASE`.
 
-**Selecting a different mbed-os tree**
+### Selecting a different Mbed OS tree
 
 ```
 cd $sketchbook/hardware/arduino-git/mbed