time::Tm::rfc3339 docs incorrectly refer to RFC 3999

[anguslees]

Easy typo fix that makes it harder than it should be to seach/grep for the right function. The docs for rfc3339 refer to a non-existent RFC 3999:
* Returns a time string formatted according to RFC 3999. RFC 3999 is
* compatible with ISO 8601.

The function is named correctly. The comment should be s/RFC 3999/RFC 3339/g

[omasanori]

Oh, it's my fault. Sorry.