Corrected spelling error in README.md

Author: mludowise

README.md

@@ -4,7 +4,7 @@ Sketch Plugin Manager
 Sketch Plugin Manager uses the power of Git to keep all of your installed plugins up to date.
 
 ### Supported Operating Systems
-Sketch Plugin Manager is currently only supported on **OS X 10.12 Seirra**. A build for 10.11 El Capitan is in the works. To get notified of changes, **watch** this [project on GitHub](https://github.com/mludowise/Sketch-Plugin-Manager).
+Sketch Plugin Manager is currently only supported on **OS X 10.12 Sierra**. A build for 10.11 El Capitan is in the works. To get notified of changes, **watch** this [project on GitHub](https://github.com/mludowise/Sketch-Plugin-Manager).
 
 Usage
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