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>

Documentation/git.txt

@@ -493,7 +493,7 @@ double-quotes and respecting backslash escapes. E.g., the value
 	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