Update README.md

Author: xiaohaoliang

tools/run_tests/README.md

@@ -33,7 +33,7 @@ the script also supports orchestrating test runs with client and server running
 to BigQuery.
 
 ######Example
-`tools/run_tests/run_peformance_tests.py -l c++ node`
+`tools/run_tests/run_performance_tests.py -l c++ node`
 
 ######Useful options
 - `--regex` use regex to select particular scenarios to run.