C comment: correct use of 64-"byte" cache line size

Reported-by: Kelly Min

Discussion: https://postgr.es/m/CAPSbxatOiQO90LYpSC3+svAU9-sHgDfEP4oFhcEUt_X=DqFA9g@mail.gmail.com

Backpatch-through: 9.5

src/include/storage/buf_internals.h

@@ -203,7 +203,7 @@ typedef struct BufferDesc
  * Note that local buffer descriptors aren't forced to be aligned - as there's
  * no concurrent access to those it's unlikely to be beneficial.
  *
- * We use 64bit as the cache line size here, because that's the most common
+ * We use a 64-byte cache line size here, because that's the most common
  * size. Making it bigger would be a waste of memory. Even if running on a
  * platform with either 32 or 128 byte line sizes, it's good to align to
  * boundaries and avoid false sharing.