fixed a typo (home-assistant#982)

JSprengard · Landrash · commit f3d247987ed9 · 2016-09-24T17:53:47.000+02:00
Fix of typos in development_environment.markdown by @JSprengard
diff --git a/source/developers/development_environment.markdown b/source/developers/development_environment.markdown
@@ -27,7 +27,7 @@ After following these steps, running `hass` will invoke your local installation.
 ###Developing on Windows 
 If you are using Windows as a development platform ensure you have the correct Microsoft Visual C++ build tools installed. Please check [the Windows Compilers](https://wiki.python.org/moin/WindowsCompilers) section on the [Python website](https://www.python.org/) for details. Validation using `tox` will fail if this is not done correctly. 
 
-Ensure you install or upgrade the Setuptools Python package. It contain compatibility improvements and add automatic use of compilers: 
+Ensure you install or upgrade the Setuptools Python package. It contains compatibility improvements and adds automatic use of compilers: 
 ```bash
 pip install --upgrade setuptools
 ```
