When using "stty susp ''" to disable sending the TSTP signal with ^Z,
mpv didn't recognize ^Z correctly in the terminal:
[input] No key binding found for key 'Ctrl+2'.
Because ASCII 26 (^Z) and above were incorrectly considered ^<NUMBER>.
This commit moves the cutoff between letters/numbers from 25 to 26 so
that ^Z is now detected correctly as ^<LETTER>.
Additionally, it rephrases the ^<NUMBER> formula for clarity.