[whatwg] Minor linking addition to parsing section

[James Graham]

It would be useful if in section 9.2.4.4. "The trailing end phase", 
phrases such as "Switch back to the main phase and reprocess the token." 
linked to the part of section 9.2.4.3.6. "The insertion mode" that 
define which insertion mode the main phase should be in when making this 
switch.

-- 
"The universe doesn't care what you believe. The wonderful thing about 
science is that it doesn't ask for your faith, it just asks for your 
eyes" --- http://xkcd.com/c154.html