Update ui_spectrum.cpp

I changed width in pixels of the "fine-tune cursor" from 2 to 5 , and then re-centered the cursor, from 120 to 118 to accomodate the shift in width.

I was inspired by this old ISSUE on Havoc's repository, where at the end @furrtek commented the need to make the red tick bigger in the future (but forgot / was swamped with other ehnancements / issues):

https://github.com/furrtek/portapack-havoc/issues/172