index-format: use 'cache tree' over 'cached tree'

index-format: use 'cache tree' over 'cached tree'

The index has a "cache tree" extension. This corresponds to a
significant API implemented in cache-tree.[ch]. However, there are a few
places that refer to this erroneously as "cached tree". These are rare,
but notably the index-format.txt file itself makes this error.

The only other reference is in t7104-reset-hard.sh.

Reported-by: Junio C Hamano <redacted>
Signed-off-by: Derrick Stolee <redacted>
Signed-off-by: Junio C Hamano <redacted>