Minor English Tweak

Replace `a` with `an`

README.md

@@ -81,7 +81,7 @@ You can view more in [the list on the wiki](https://github.com/tripit/slate/wiki
 Need Help? Found a bug?
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-Read our [contribution guidelines](https://github.com/tripit/slate/blob/master/CONTRIBUTING.md), and then [submit a issue](https://github.com/tripit/slate/issues) to the Slate Github if you need any help. And, of course, feel free to submit pull requests with bug fixes or changes.
+Read our [contribution guidelines](https://github.com/tripit/slate/blob/master/CONTRIBUTING.md), and then [submit an issue](https://github.com/tripit/slate/issues) to the Slate Github if you need any help. And, of course, feel free to submit pull requests with bug fixes or changes.
 
 Contributors
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