Use imperative tone for suggested commit message (#2050)

This is both widely recommended as the preferred style and also that in which actual commit messages on this project are written.

README.md

@@ -49,7 +49,7 @@ The documentation is divided into several sections with a different tone and pur
 
 ### Push it
 
-1. `git add -A && git commit -m "My message"` (replacing `My message` with a commit message, such as `Fixed header logo on Android`) to stage and commit your changes
+1. `git add -A && git commit -m "My message"` (replacing `My message` with a commit message, such as `Fix header logo on Android`) to stage and commit your changes
 1. `git push my-fork-name the-name-of-my-branch`
 1. Go to the [reactjs.org repo](https://github.com/reactjs/reactjs.org) and you should see recently pushed branches.
 1. Follow GitHub's instructions.