Fix the incorrect Jest watch command in Contributing

Author: gaearon

CONTRIBUTING.md

@@ -22,9 +22,9 @@ This command is just a facade to [Jest](https://facebook.github.io/jest/). You m
 
 For example, `jest --watch` lets you automatically run the test suite on every file change.
 
-You can also run a subset of tests by passing a prefix to `jest`. For example, `jest ReactDOMSVG` will only run tests in the files that start with `ReactDOMSVG`, such as `ReactDOMSVG-test.js`.
+You can also run a subset of tests by passing a prefix to `jest`. For instance, `jest ReactDOMSVG` will only run tests in the files that start with `ReactDOMSVG`, such as `ReactDOMSVG-test.js`.
 
-When you know which tests you want to run, you can achieve a fast feedback loop by using these two features together. For example, `jest --watch ReactDOMSVG` will re-run only the matching tests on every change.
+When you know which tests you want to run, you can achieve a fast feedback loop by using these two features together. For example, `jest ReactDOMSVG --watch` will re-run only the matching tests on every change.
 
 Just make sure to run the whole test suite before submitting a pull request!