v5.0.x update readme, point to v5.0.0 documentation link

Signed-off-by: Tomislav Janjusic <[email protected]>

README.md

@@ -14,7 +14,7 @@ application developers and computer science researchers.
 
 The Open MPI documentation can be viewed in the following ways:
 
-1. Online at https://docs.open-mpi.org/
+1. Online at https://docs.open-mpi.org/en/v5.0.0/
 1. In self-contained (i.e., suitable for local viewing, without an
    internet connection) in official distribution tarballs under
    `docs/_build/html/index.html`.