docs: arch is gone, use delpart as usage() function example

The arch.c was removed in commit 27abd809.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>

Documentation/howto-usage-function.txt

@@ -153,5 +153,5 @@ are meant this.
 Example
 -------
 
-Command sys-utils/arch.c is a minimal example how to do write usage
+Command disk-utils/delpart.c is a minimal example how to do write usage
 function, setup option parsing, version printing and so on.

include/c.h

@@ -262,7 +262,7 @@ static inline int usleep(useconds_t usec)
 
 /*
  * Constant strings for usage() functions. For more info see
- * Documentation/howto-usage-function.txt and sys-utils/arch.c
+ * Documentation/howto-usage-function.txt and disk-utils/delpart.c
  */
 #define USAGE_HEADER     _("\nUsage:\n")
 #define USAGE_OPTIONS    _("\nOptions:\n")