dotdot.c: Fix a RFC section number in a comment for Curl_dedotdotify()

 *

 * This function gets a zero-terminated path with dot and dotdot sequences

 * passed in and strips them off according to the rules in RFC 3986 section

 * 5.2.5.

 * 5.2.4.

 *

 * The function handles a query part ('?' + stuff) appended but it expects

 * that fragments ('#' + stuff) have already been cut off.