lineNumber has casing that is inconsistent with surrounding options,
like color.grep.matchContext, and color.grep.matchSelected. Re-case this
documentation in order to be consistent with the text around it, and to
ensure that new entries are consistent, too.
Signed-off-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>