Fix incorrect output in example

Fixes #1299

Author: zoffixznet

doc/Language/rb-nutshell.pod6

@@ -974,7 +974,7 @@ the C<MAIN> subroutine.
         Verbosity on
     perl6 example.p6 --length=abc
         Usage:
-          c.p6 [--length=<Int>] [--file=<Any>] [--verbose]
+          example.p6 [--length=<Int>] [--file=<Any>] [--verbose]
 
 Note that Perl 6 auto-generates a full usage message on error in
 command-line parsing.