sub-process: correct path to API docs in a comment

Signed-off-by: NChristian Couder <chriscool@tuxfamily.org>
Acked-by: NBen Peart <peartben@gmail.com>
Reviewed-by: NJonathan Nieder <jrnieder@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

sub-process.h

@@ -7,7 +7,7 @@
 
 /*
  * Generic implementation of background process infrastructure.
- * See Documentation/technical/api-background-process.txt.
+ * See: Documentation/technical/api-sub-process.txt
  */
 
  /* data structures */