Update hints for Gatekeeper 1 (OpenZeppelin#212)

Co-authored-by: Francisco Giordano <[email protected]>

gamedata/descriptions/levels/gatekeeper1.md

@@ -2,4 +2,4 @@ Make it past the gatekeeper and register as an entrant to pass this level.
 
 ##### Things that might help:
 * Remember what you've learned from the Telephone and Token levels.
-* You can learn more about the `msg.gas` special variable, or its preferred alias `gasleft()`, in Solidity's documentation (see [here](http://solidity.readthedocs.io/en/v0.4.23/units-and-global-variables.html) and [here](http://solidity.readthedocs.io/en/v0.4.23/control-structures.html#external-function-calls)).
+* You can learn more about the special function `gasleft()`, in Solidity's documentation (see [here](https://docs.soliditylang.org/en/v0.8.3/units-and-global-variables.html) and [here](https://docs.soliditylang.org/en/v0.8.3/control-structures.html#external-function-calls)).