README: fix instructions for running tests

Author: kemenaran

README.md

@@ -62,7 +62,7 @@ By default, the siret validator may emit two different error messages:
 
 ## Development
 
-After checking out the repo, run `bin/setup` to install dependencies. Then, run `rake test` to run the tests.
+After checking out the repo, run `bin/setup` to install dependencies. Then, run `bundle exec rake test` to run the tests.
 
 To install this gem onto your local machine, run `bundle exec rake install`. To release a new version, update the version number in `siret_validator.gemspec`, and then run `bundle exec rake release`, which will create a git tag for the version, push git commits and the created tag, and push the `.gem` file to [rubygems.org](https://rubygems.org).