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 <gitster@pobox.com>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/index-format.txt

@@ -26,7 +26,7 @@ Git index format
      Extensions are identified by signature. Optional extensions can
      be ignored if Git does not understand them.
 
-     Git currently supports cached tree and resolve undo extensions.
+     Git currently supports cache tree and resolve undo extensions.
 
      4-byte extension signature. If the first byte is 'A'..'Z' the
      extension is optional and can be ignored.
@@ -136,9 +136,9 @@ Git index format
 
 == Extensions
 
-=== Cached tree
+=== Cache tree
 
-  Cached tree extension contains pre-computed hashes for trees that can
+  Cache tree extension contains pre-computed hashes for trees that can
   be derived from the index. It helps speed up tree object generation
   from index for a new commit.
 

t/t7104-reset-hard.sh

@@ -33,7 +33,7 @@ test_expect_success 'reset --hard should restore unmerged ones' '
 
 '
 
-test_expect_success 'reset --hard did not corrupt index or cached-tree' '
+test_expect_success 'reset --hard did not corrupt index or cache-tree' '
 
 	T=$(git write-tree) &&
 	rm -f .git/index &&