Update one-time-code in README

[stripedpajamas]

Looks like there's already a PR in rmcl to update the one-time-code link (rschroll/rmcl#8). This PR updates the README which references a dead link.

[nils-werner]

There is also an URL in the code that gets shown upon first run, could you update that one too?

[ec1oud]

But it should be updated in the code too:

$ rmfuse /mnt/remarkable
This reMarkable client needs to be registered with the reMarkable
cloud. To do this, please visit
    https://my.remarkable.com/connect/desktop
to get a one-time code.

(You may be prompted to log into your reMarkable cloud account. If
this happens, you may not be redirected to the one-time code page.
In this case, you may open the above link a second time to get the
code.)

Enter the one-time code: 

[tlaundal]

I think the URL might have been changed again. https://my.remarkable.com/#desktop now redirects to https://my.remarkable.com/device/desktop/connect
See also #30