Update RTEMS documentation url (fixes #211)

Author: Bckempa

rtems/README.md

@@ -1,5 +1,5 @@
 # RTEMS
-This folder contains a Dockerfile that build RTEMS from source following the [official doc](https://docs.rtems.org/branches/master/user/overview/index.html).
+This folder contains a Dockerfile that build RTEMS from source following the [official docs](https://docs.rtems.org/docs/main/user/overview/index.html).
 The target platform for the build is leon3.
 This will not run RTEMS, just perform the build and set up tools.