op-e2e: Add forge clean to README.md troubleshooting section (ethereum-optimism#11471)

* op-e2e: Add forge clean to README.md troubleshooting section

* Update op-e2e/README.md

Co-authored-by: Matt Solomon <[email protected]>

---------

Co-authored-by: Matt Solomon <[email protected]>

Author: sebastianst

op-e2e/README.md

@@ -31,4 +31,4 @@ make test-http
 If you encounter errors:
 * ensure you have the latest version of foundry installed: `just update-foundry`
 * try deleting the `packages/contracts-bedrock/forge-artifacts` directory
-* if the above step doesn't fix the error, try `pnpm clean`
+* try `forge clean && rm -rf lib && forge install` within the `packages/contracts-bedrock` directory