docs: To build OVS on RHEL7 EPEL is needed

Since Python 3 is now mandatory, Extra Packages for Enterprise Linux
(EPEL) repository is needed in order to build OVS on RHEL7.

Signed-off-by: Timothy Redaelli <tredaelli@redhat.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>

Documentation/intro/install/fedora.rst

@@ -63,11 +63,12 @@ The command below will create a temporary SPEC file::
       > /tmp/ovs.spec
 
 And to install specific dependencies, use the corresponding tool below.
-For some of the dependencies on RHEL you may need to add two additional
+For some of the dependencies on RHEL 7 you may need to add three additional
 repositories to help yum-builddep, e.g.::
 
     $ subscription-manager repos --enable=rhel-7-server-extras-rpms
     $ subscription-manager repos --enable=rhel-7-server-optional-rpms
+    $ yum install https://dl.fedoraproject.org/pub/epel/epel-release-latest-7.noarch.rpm
 
 DNF::