Fix spelling in readme.md

Change UK to international spelling.

Author: Amanda Butler

readme.md

@@ -1,11 +1,12 @@
 # Mbed OS 5 documentation
 
-This is the documentation source for Mbed OS 5. It's optimised to be read on [the documentation site](https://os.mbed.com/docs/latest/).
+This is the documentation source for Mbed OS 5. It's optimized to be read on [the documentation site](https://os.mbed.com/docs/latest/).
 
 If you want to contribute, please read our [contribution guide](https://os.mbed.com/docs/latest/contributing/index.html).
 
 Please open pull requests against either:
 
 - `development` - this is our development branch.
 - `5.11` - this is the latest live version.
-- Older version branches probably don't require fixes; please open an issue if you think we're wrong.
+
+Older version branches probably don't require fixes; please open an issue if you think we're wrong.