Minor typos fixed in guide.md

Very minor.  My playing with different markdown apps got me to read the spec enough to know how to fix "_req.body._method_" to "_req.body.\_method_" in order to get the embedded '_' to work with discount.

Check the mime 'type' <--> 'subtype' reversal.  I'm pretty sure I've got it right.  (section `req.is()` approx. line 685)

I'll have to check some stuff before next pull request and consult with you whether I'm right / wrong / _other_ ...