ref_iterator_begin_fn(): fix docstring

ref_iterator_begin_fn(): fix docstring

The iterator returned by this function only includes references whose
names start with the whole prefix, not all of those in
`find_containing_dir(prefix)` as the old docstring claimed. This
docstring was probably copy-pasted from old ref-cache code, which had
the old specification. But now, `cache_ref_iterator_begin()`
(from which the files reference iterator gets its values)
automatically wraps its output using `prefix_ref_iterator_begin()`
when necessary, so it has the stricter behavior.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>