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
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}