correct mistyped msg. (#4341)

Summary:
corrected the mistyped message.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4341

Differential Revision: D9816571

Pulled By: ajkr

fbshipit-source-id: 1df0424e981a01470a638a37b925c4133d59a48b

tools/write_external_sst.sh

@@ -4,7 +4,7 @@
 #
 
 if [ "$#" -lt 3 ]; then
-  echo "usagee: $BASH_SOURCE <input_data_path> <DB Path> <extern SST dir>"
+  echo "usage: $BASH_SOURCE <input_data_path> <DB Path> <extern SST dir>"
   exit 1
 fi