t3060: fix mention of function prune_index

t3060: fix mention of function prune_index

Commit [1] added tests which trigger function prune_cache.  The comments
in these tests, however, incorrectly call it "prune_path".  Since then,
function "prune_cache" has been renamed to "prune_index" in commit [2].
Later still in commit [3], the_index singleton, which is also mentioned
in a comment, stopped being used directly with function "prune_index".

Fix mentions of function "prune_index" and the struct it changes in
comments in file "t3060-ls-files-with-tree.sh".

[1] 54e1abce90 (Add test case for ls-files --with-tree, 2007-10-03)
[2] 6510ae173a (ls-files: convert prune_cache to take an index,
    2017-06-12)
[3] 188dce131f (ls-files: use repository object, 2017-06-22)

Signed-off-by: Andrei Rybak <rybak.a.v@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>