fastboot/README.md: Update response packet max size to 256

This change is to update the doc to reflect the current state and it
doesn't affect the runtime behavior.
The max packet size was updated to 256 by aosp/1226715.

Bug: 196676392
Test: Presubmit; doc update doesn't affect build
Change-Id: I73b0a416730d09fdb24ea81bffe52ddd89534132
1 file changed