Update links in Lighthouse Book (#4279)

## Issue Addressed

Update some broken links in Lighthouse Book

## Proposed Changes

The updated links correctly link to the section of the book

## Additional Info

Please provide any additional information. For example, future considerations
or information useful for reviewers.

book/src/mainnet-validator.md

@@ -2,7 +2,6 @@
 
 [launchpad]: https://launchpad.ethereum.org/
 [lh-book]: https://lighthouse-book.sigmaprime.io/
-[testnet-validator]: ./testnet-validator.md
 [advanced-datadir]: ./advanced-datadir.md
 [license]: https://github.com/sigp/lighthouse/blob/stable/LICENSE
 [slashing]: ./slashing-protection.md
@@ -15,7 +14,7 @@ managing servers. You'll also need at least 32 ETH!
 Being educated is critical to a validator's success. Before submitting your mainnet deposit, we recommend:
 
 - Thoroughly exploring the [Staking Launchpad][launchpad] website, try running through the deposit process using a testnet launchpad such as the [Goerli staking launchpad](https://goerli.launchpad.ethereum.org/en/).
-- Running a [testnet validator][testnet-validator].
+- Running a testnet validator.
 - Reading through this documentation, especially the [Slashing Protection][slashing] section.
 - Performing a web search and doing your own research.
 

book/src/run_a_node.md

@@ -6,7 +6,7 @@ You should be finished with one [Installation](./installation.md) method of your
 1. Create a [JWT secret file](#step-1-create-a-jwt-secret-file)
 1. Set up an [execution node](#step-2-set-up-an-execution-node);
 1. Set up a [beacon node](#step-3-set-up-a-beacon-node-using-lighthouse);
-1. [Check logs for sync status](#step-4-check-logs);
+1. [Check logs for sync status](#step-4-check-logs-for-sync-status);