Fixed typo

Author: benjiwheeler

README.md

@@ -74,7 +74,7 @@ See [jest cli docs](https://facebook.github.io/jest/docs/en/cli.html#content) fo
 
 *NOTE: If you're a windows user, please run these scripts in Windows `cmd.exe`  instead of Git Bash/MINGW64.*
 
-Before running any test, make sure you have run `npm install` from this (scratch-gui) repository's top level.
+Before running any tests, make sure you have run `npm install` from this (scratch-gui) repository's top level.
 
 #### Main testing command