Fix readme formatting

Author: mrudzki

README.md

@@ -96,7 +96,7 @@ Options:
         --runtime-log FILE           Location of previously recorded test runtimes
     -v, --verbose                    More output
         --fail-fast=[N]
-    --print_failed_group             Prints group that had failures in it
+        --print_failed_group         Prints group that had failures in it
 ```
 
 ## Development