fix(docs): typo in the romlib design

There's a typo in the romlib design document when referring to
the generator script. It should be romlib_generator.py instead
of romlib_generate.py so fixed this typo.

Change-Id: I6db7ee66b13c2b0b9d8511da7e0d1b058366281b
Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>