Add missing whitespace.

Summary: Closes https://github.com/facebook/rocksdb/pull/3729

Differential Revision: D7645465

Pulled By: riversand963

fbshipit-source-id: a64da0960fe6c39847ef848b8888fe9a9c1df25d

tools/db_bench_tool.cc

@@ -299,7 +299,7 @@ DEFINE_int64(write_buffer_size, rocksdb::Options().write_buffer_size,
 DEFINE_int32(max_write_buffer_number,
              rocksdb::Options().max_write_buffer_number,
              "The number of in-memory memtables. Each memtable is of size"
-             "write_buffer_size.");
+             " write_buffer_size bytes.");
 
 DEFINE_int32(min_write_buffer_number_to_merge,
              rocksdb::Options().min_write_buffer_number_to_merge,