perlapi: Fix typo: 1 should have been -1

Author: khwilliamson

utf8.c

@@ -1069,7 +1069,7 @@ C<retlen> to C<-1> (cast to C<STRLEN>) and return zero.
 Note that this API requires disambiguation between successful decoding a C<NUL>
 character, and an error return (unless the C<UTF8_CHECK_ONLY> flag is set), as
 in both cases, 0 is returned, and, depending on the malformation, C<retlen> may
-be set to 1.  To disambiguate, upon a zero return, see if the first byte of
+be set to -1.  To disambiguate, upon a zero return, see if the first byte of
 C<s> is 0 as well.  If so, the input was a C<NUL>; if not, the input had an
 error.  Or you can use C<L</utf8n_to_uvchr_error>>.