update CayenneLPP docs link

[asanchezdelc]

We recently made a few changes to our docs page, so this change is reflecting that.

[CLAassistant]

All committers have signed the CLA.

[jpmeijers]

LGTM

I see in your docs you refer to Cayenne Low Power Payload (LPP), while we talk about Cayenne Lower Power Protocol (LPP). I take it that we should update that too.

[johanstokking]

@asanchezdelc can you sign CLA and update to using Low Power Payload?

[asanchezdelc]

@johanstokking done.

[johanstokking]

Thanks!