GDAX deprecation: switch docs URLs to docs.pro.coinbase.com.
GDAX deprecation: switch docs URLs to docs.pro.coinbase.com.

$ perl -w -p -i -e 's#https://docs\.gdax\.com#https://docs.pro.coinbase.com#g' *

Pick docs.pro.coinbase.com instead of docs.prime.coinbase.com because
this module is much more likely to be used by individual developers
rather than financial institutions [1].

[1] https://support.pro.coinbase.com/customer/en/portal/articles/2945306-what-is-the-difference-between-pro-and-prime-

coinbase/gdax-node