Fix typo in CacheStats.h (#383)

Summary: Pull Request resolved: #383

Reviewed By: pbhandar2

Differential Revision: D73636353

Pulled By: haowu14

fbshipit-source-id: 040a027ca3046b5e2ade86e56d71617ebb5e03ae

Author: borislin

cachelib/allocator/CacheStats.h

@@ -604,7 +604,7 @@ struct CacheMemoryStats {
   // returns the advised memory in the unit of slabs.
   size_t numAdvisedSlabs() const { return advisedSize / Slab::kSize; }
 
-  // returne usable portion of the cache size
+  // returns usable portion of the cache size
   size_t usableRamCacheSize() const { return ramCacheSize; }
 };