Correct name of diff_flush() in API documentation

Signed-off-by: Daniel Barkalow <barkalow@iabervon.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/api-diff.txt

@@ -39,7 +39,7 @@ Calling sequence
 * Once you finish feeding the pairs of files, call `diffcore_std()`.
   This will tell the diffcore library to go ahead and do its work.
 
-* Calling `diffcore_flush()` will produce the output.
+* Calling `diff_flush()` will produce the output.
 
 
 Data structures