Typo in MMU2 serial protocol description (#19949)

Marlin/src/feature/mmu2/serial-protocol.md

 
 - *Firmware version* is an integer value, but we don't care about it
 - *Build number* is an integer value and has to be >=126, or =>132 if 12V mode is enabled
-- *FINDA status* is 1 if the is filament loaded to the extruder, 0 otherwise
+- *FINDA status* is 1 if the filament is loaded to the extruder, 0 otherwise
 
 
 *Build number* is checked against the required value, if it does not match, printer is halted.