Add clarification comment in SparseFileContent.Range.toString

Change I34fb07f incorrectly "fixed" the brackets in the `toString`
method, however the usage of [ and ) was intentional, to denote that
the range is inclusive/exclusive.

Add a short comment in the method to prevent the same mistake from
being made again.

Change-Id: Idfe9b560c0cc55bfa4a6ab06fba172d971c3089c
1 file changed