fix comment

Summary:
Signed-off-by: Ntang.jin <tang.jin@istuary.com>
Closes https://github.com/facebook/rocksdb/pull/2644

Differential Revision: D5600861

Pulled By: yiwu-arbug

fbshipit-source-id: 9516636cb6e77b09fe0ebef78953adf4b7e88cc8

db/write_thread.h

@@ -55,7 +55,7 @@ class WriteThread {
 
     // The state used to inform a waiting writer that it has become a
     // parallel memtable writer. It can be the group leader who launch the
-    // praallel writer group, or one of the followers. The writer should then
+    // parallel writer group, or one of the followers. The writer should then
     // apply its batch to the memtable concurrently and call
     // CompleteParallelMemTableWriter.
     STATE_PARALLEL_MEMTABLE_WRITER = 8,