Update servo control vesc instructions

[ajak2k]

Describe the bug
In 'getting_started/firmware/firmware_vesc.rst' line 59 (steps for enabling servo output). Users need to also click on 'Write App Configuration' (highlighted in the screenshot below) to enable servo output in the hardware. This instruction seems to be missed and needs to be added

Screenshots

1. Screenshot of current instructions on the f1tenth build documentation here

2. Screenshot from VESC tool 6.02 highlighting the 'Write App Configuration' button