Fix links to bouncers guide

(cherry picked from commit d8a25d17aafbd78c40e18396ed8f18fd40daa26e)

contracts/GSN/README.adoc

@@ -2,7 +2,7 @@
 
 NOTE: This feature is being released in the next version of OpenZeppelin Contracts, available right now through `npm install @openzeppelin/contracts@next`.
 
-TIP: Check out our guide on the xref:ROOT:gsn.adoc[basics of the GSN] as well as the xref:ROOT:gsn-advanced.adoc[more advanced topics].
+TIP: Check out our guide on the xref:ROOT:gsn.adoc[basics of the GSN] as well as the xref:ROOT:gsn-bouncers.adoc[more advanced topics].
 
 == Recipient
 

contracts/GSN/bouncers/GSNBouncerERC20Fee.sol

@@ -8,7 +8,7 @@ import "../../token/ERC20/ERC20.sol";
 import "../../token/ERC20/ERC20Detailed.sol";
 
 /**
- * @dev A xref:ROOT:gsn-advanced.adoc#gsn-bouncers[GSN Bouncer] that charges transaction fees in a special purpose ERC20
+ * @dev A xref:ROOT:gsn-bouncers.adoc#gsn-bouncers[GSN Bouncer] that charges transaction fees in a special purpose ERC20
  * token, which we refer to as the gas payment token. The amount charged is exactly the amount of Ether charged to the
  * recipient. This means that the token is essentially pegged to the value of Ether.
  *

contracts/GSN/bouncers/GSNBouncerSignature.sol

@@ -4,7 +4,7 @@ import "./GSNBouncerBase.sol";
 import "../../cryptography/ECDSA.sol";
 
 /**
- * @dev A xref:ROOT:gsn-advanced.adoc#gsn-bouncers[GSN Bouncer] that allows relayed transactions through when they are
+ * @dev A xref:ROOT:gsn-bouncers.adoc#gsn-bouncers[GSN Bouncer] that allows relayed transactions through when they are
  * accompanied by the signature of a trusted signer. The intent is for this signature to be generated by a server that
  * performs validations off-chain. Note that nothing is charged to the user in this scheme. Thus, the server should make
  * sure to account for this in their economic and threat model.