miscellaneous typo fixes

One thought I left out (because I think this will be better stated in the `implicit` chapter):

```
For OAuth flows where the client secret cannot be used, such as in
the Implicit grant type in the next chapter, registering the ``redirect_uri`` 
is critical in establishing a trusted exchange with the client.
```

I thought of putting this in the **Registering the Redirect URI** section, but it sounded out of place.

The rest are typo fixes and some optional links I added for convenience (although, it seems you may have left these out on purpose?)