Fix B->Xll sprucing lines filter codes functions

Fix to function in B->Xll sprucing lines which parses line name and produces filter codes (introduced in !2431 (merged)), comma missing in https://gitlab.cern.ch/lhcb/Moore/-/blame/master/Hlt/Hlt2Conf/python/Hlt2Conf/lines/rd/b_to_xll_spruce.py#L109 (thanks to @matzeni for spotting this 🔍). Function has also been renamed get_hlt2_filter_code -> parse_hlt2_filter_code for clarity.

FYI @rmwillia @nskidmor