Documentation: correct hash environment variable

To set the default hash algorithm you can set the `GIT_DEFAULT_HASH`
environment variable. In the documentation this variable is named
`GIT_DEFAULT_HASH_ALGORITHM`, which is incorrect.

Signed-off-by: Toon Claes <toon@iotcl.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/Documentation/git.txt b/Documentation/git.txt
index 9d6769e..1289084 100644
--- a/Documentation/git.txt
+++ b/Documentation/git.txt
@@ -493,7 +493,7 @@
 	details. This variable has lower precedence than other path
 	variables such as GIT_INDEX_FILE, GIT_OBJECT_DIRECTORY...
 
-`GIT_DEFAULT_HASH_ALGORITHM`::
+`GIT_DEFAULT_HASH`::
 	If this variable is set, the default hash algorithm for new
 	repositories will be set to this value. This value is currently
 	ignored when cloning; the setting of the remote repository