Add solidity language to missing code snippets (#3992)

contracts/utils/structs/DoubleEndedQueue.sol

@@ -12,7 +12,7 @@ import "../math/SafeCast.sol";
  *
  * The struct is called `Bytes32Deque`. Other types can be cast to and from `bytes32`. This data structure can only be
  * used in storage, and not in memory.
- * ```
+ * ```solidity
  * DoubleEndedQueue.Bytes32Deque queue;
  * ```
  *

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;

contracts/utils/structs/EnumerableSet.sol

@@ -15,7 +15,7 @@ pragma solidity ^0.8.0;
  * (O(1)).
  * - Elements are enumerated in O(n). No guarantees are made on the ordering.
  *
- * ```
+ * ```solidity
  * contract Example {
  *     // Add the library methods
  *     using EnumerableSet for EnumerableSet.AddressSet;