Add solidity language to missing code snippets (#3992)

contracts/utils/structs/EnumerableMap.sol

@@ -17,7 +17,7 @@ import "./EnumerableSet.sol";
  * (O(1)).
  * - Entries are enumerated in O(n). No guarantees are made on the ordering.
  *
- * ```
+ * ```solidity
  * contract Example {
  *     // Add the library methods
  *     using EnumerableMap for EnumerableMap.UintToAddressMap;