Fix typos in comments (#7687)

Summary:
Hi there,

This PR fixes a few typos in comments in `cache/lru_cache.h`.

Thanks

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7687

Reviewed By: ajkr

Differential Revision: D25064674

Pulled By: jay-zhuang

fbshipit-source-id: fe633369d5b82c5aac42d4ee8d551b9d657237d1

cache/lru_cache.h

@@ -67,7 +67,7 @@ struct LRUHandle {
     IS_HIGH_PRI = (1 << 1),
     // Whether this entry is in high-pri pool.
     IN_HIGH_PRI_POOL = (1 << 2),
-    // Wwhether this entry has had any lookups (hits).
+    // Whether this entry has had any lookups (hits).
     HAS_HIT = (1 << 3),
   };
 
@@ -130,7 +130,7 @@ struct LRUHandle {
     delete[] reinterpret_cast<char*>(this);
   }
 
-  // Caclculate the memory usage by metadata
+  // Calculate the memory usage by metadata
   inline size_t CalcTotalCharge(
       CacheMetadataChargePolicy metadata_charge_policy) {
     size_t meta_charge = 0;