releasetools: RangeSet.monotonic is not an optional attribute.

'monotonic' has been non-optional since [1] (L-MR1). Fix the comment in
RangeSet.parse(), as well as the use in blockimgdiff.py.

[1] commit 8b72aefb.

Test: Generate an incremental BBOTA package.
Change-Id: I7f95231683473b4f0f07f9c83fccc0e36a1340cb