[Edit docs] Small link fix for mining page

[ecorona9]

URL of the page to change: https://docs.stacks.co/docs/stacks-academy/mining

Explain here what you would like to change.

The clickable link called miner on mainnet as below

https://docs.stacks.co/docs/nodes-and-miners/miner-testnet

should instead take you to

https://docs.stacks.co/docs/nodes-and-miners/miner-mainnet