Edit CONTRIBUTING.md

Edit file, mostly for minor comma nits.

Author: Amanda Butler

CONTRIBUTING.md

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 # Contributing to Mbed OS
 
-Mbed OS is an open source, device software, platform for the internet of things. Contributions are an important part of the platform and our goal is to make it as simple as possible to become a contributor.  
+Mbed OS is an open-source, device software platform for the internet of things. Contributions are an important part of the platform, and our goal is to make it as simple as possible to become a contributor.
 
-To encourage productive collaboration, as well as robust, consistent, and maintainable code, we have built up a set of guidelines for contributing to Mbed OS. Please see: [contributing guidelines](https://os.mbed.com/docs/latest/reference/contributing.html)
+To encourage productive collaboration, as well as robust, consistent and maintainable code, we have built a set of guidelines for contributing to Mbed OS. Please see: [contributing guidelines](https://os.mbed.com/docs/latest/reference/contributing.html).