The HTTP/2 spec is done on https://github.com/http2/http2-spec, this particular change seem to be quietly introduced here:

    commit ac468f3fab9f7092a430eedfd69ee1fb2e23c944
    Author: Martin Thomson <martin.thomson@gmail.com>
    Date:   Fri Jun 14 13:14:02 2013 -0700

        Exercising editorial discretion regarding magic.

     diff --git a/draft-ietf-httpbis-http2.xml b/draft-ietf-httpbis-http2.xml
    index 58bbc27..f1e570d 100644
    --- a/draft-ietf-httpbis-http2.xml
    +++ b/draft-ietf-httpbis-http2.xml
    @@ -385,9 +385,9 @@ Upgrade: HTTP/2.0
               The client connection header is a sequence of 24 octets (in hex notation)
             </t>
             <figure><artwork type="inline">
    -535441202a20485454502f322e300d0a0d0a52540d0a0d0a</artwork></figure>
    +505249202a20485454502f322e300d0a0d0a534d0d0a0d0a</artwork></figure>
             <t>
    -          (the string <spanx style="verb">STA * HTTP/2.0\r\n\r\nRT\r\n\r\n</spanx>) followed by a
    +          (the string <spanx style="verb">PRI * HTTP/2.0\r\n\r\nSM\r\n\r\n</spanx>) followed by a
               <xref target="SETTINGS">SETTINGS frame</xref>.  The client sends the client connection header
               immediately upon receipt of a 101 Switching Protocols response (indicating a successful
               upgrade), or after receiving a TLS Finished message from the server. If starting an