[BUG] Update the references to mainnet genesis file after a recent refactoring to nearcore

[frol]

After this PR near/nearcore#4292, the folder should be nearcore/res/

[thisisjoshford]

@frol Before I saw this issue just now, I noticed some broken links due to this refactor and started fixing them here ( #728
& #726 ). The solution I implemented was to tell people how to query this data via the protocol_config endpoint rather than linking to the config file directly. Should I continue on this path for the remaining links above in gas.MD or direct link to the new location? My thoughts are that if things get refactored or updated again the RPC endpoint will show the most current data and will minimize doc maintenance.

[frol]

@thisisjoshford yeah, using the protocol_config endpoint is even better! Thanks for pointing it out! I just used grep to find the broken links, and did not really dig into the reason why those links are used in the first place. Thanks for pointing that out! Should we close this issue?