Fix merge operator docs typo (#7716)

Summary: Pull Request resolved: https://github.com/facebook/rocksdb/pull/7716

Reviewed By: pdillinger

Differential Revision: D25214340

Pulled By: zhichao-cao

fbshipit-source-id: 143a8e7d076917e60bbe6993d60ec55f33e2ab56

include/rocksdb/merge_operator.h

@@ -109,7 +109,7 @@ class MergeOperator {
     Slice& existing_operand;
   };
 
-  // This function applies a stack of merge operands in chrionological order
+  // This function applies a stack of merge operands in chronological order
   // on top of an existing value. There are two ways in which this method is
   // being used:
   // a) During Get() operation, it used to calculate the final value of a key