README.md: fix some accidental RST conversion

Looks like part of README.md accidentally got converted to RST.  Put
it back to Markdown.

Signed-off-by: Jeff Squyres <jsquyres@cisco.com>

Author: jsquyres

README.md

@@ -1,14 +1,14 @@
 # Open MPI
 
-`The Open MPI Project <https://www.open-mpi.org/>`_ is an open source
-implementation of the `Message Passing Interface (MPI) specification
-<https://www.mpi-forum.org/docs/>`_ that is developed and maintained
-by a consortium of academic, research, and industry partners.  Open
-MPI is therefore able to combine the expertise, technologies, and
-resources from all across the High Performance Computing community in
-order to build the best MPI library available.  Open MPI offers
-advantages for system and software vendors, application developers and
-computer science researchers.
+[The Open MPI Project](https://www.open-mpi.org/) is an open source
+implementation of the [Message Passing Interface (MPI)
+specification](https://www.mpi-forum.org/docs/) that is developed and
+maintained by a consortium of academic, research, and industry
+partners.  Open MPI is therefore able to combine the expertise,
+technologies, and resources from all across the High Performance
+Computing community in order to build the best MPI library available.
+Open MPI offers advantages for system and software vendors,
+application developers and computer science researchers.
 
 ## Official documentation