rune(2): use correct value for Runeerror.
 [rsc] --rw-rw-r-- M 1004252 glenda sys 3086 Mar 22 21:10 sys/man/2/rune
	/n/sourcesdump/2006/0322/plan9/sys/man/2/rune:71,77 - 
	/n/sourcesdump/2006/0323/plan9/sys/man/2/rune:71,80
	  .SM UTF
	  format,
	  .I chartorune
	- will convert to 0x80 and return 1.
	+ will convert to
	+ .B Runeerror
	+ (0xFFFD)
	+ and return 1.
	  .PP
	  .I Runelen
	  returns the number of bytes