Fixed link to release page

It looks like https://docs.mbed.com/docs/mbed-os-release-notes/en/latest/5_1/release results in a 404 page, but https://docs.mbed.com/docs/mbed-os-release-notes/en/latest/5_1/release/ works.

Author: seppestas

README.md

@@ -8,7 +8,7 @@ mbed OS accelerates the process of creating a connected product by providing a p
 
 Our current release series is mbed OS 5.1:
 
-- [Release Note](https://docs.mbed.com/docs/mbed-os-release-notes/en/latest/5_1/release)
+- [Release Note](https://docs.mbed.com/docs/mbed-os-release-notes/en/latest/5_1/release/)
 
 ## Getting Started for Developers
 
@@ -20,4 +20,4 @@ We have a getting started guide for developers using mbed OS in applications:
 
 We have a getting started guide for contributors working on mbed OS:
 
-- Have a look in the docs directory
+- Have a look in the docs directory