shallow.txt: document SHA-256 shallow format

Similar to recent commits, document that we list object names rather
than SHA-1s.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/shallow.txt

@@ -13,7 +13,7 @@ pretend as if they are root commits (e.g. "git log" traversal
 stops after showing them; "git fsck" does not complain saying
 the commits listed on their "parent" lines do not exist).
 
-Each line contains exactly one SHA-1. When read, a commit_graft
+Each line contains exactly one object name. When read, a commit_graft
 will be constructed, which has nr_parent < 0 to make it easier
 to discern from user provided grafts.