narrow: update TODO.rst now that we share format with sparse
The narrowspec format was unified with the sparse format in
f64ebe7d2259 (narrowspec: use sparse.parseconfig() to parse narrowspec
file (BC), 2018-08-03).
Differential Revision: https://phab.mercurial-scm.org/D4904
#require docutils gettext
$ $TESTDIR/check-gendoc da
checking for parse errors