Add solidity language to missing code snippets (#3992)

contracts/access/AccessControl.sol

@@ -19,14 +19,14 @@ import "../utils/introspection/ERC165.sol";
  * in the external API and be unique. The best way to achieve this is by
  * using `public constant` hash digests:
  *
- * ```
+ * ```solidity
  * bytes32 public constant MY_ROLE = keccak256("MY_ROLE");
  * ```
  *
  * Roles can be used to represent a set of permissions. To restrict access to a
  * function call, use {hasRole}:
  *
- * ```
+ * ```solidity
  * function foo() public {
  *     require(hasRole(MY_ROLE, msg.sender));
  *     ...