correct doc about save color pattern hid command

BEEF and CAFE are required to save pattern in memory :)

if you put 0x00 in byte 2, 3, 4 and 5 (looks like) nothing happen and the pattern is not saved.
These 0xBE, 0xEF, 0xCA, 0xFE values are those used in the command line tool.
Merge pull request #149 from avernois/save_pattern_hid_command

correct doc about save color pattern hid command
thanks!