conf.py: Fix flake8 error in build.

In commit 8fcbcca it made the line > 79, which causes build error.

Fixes: 8fcbcca ("docs: update various OVS references to OVN")
Cc: Dan Williams <dcbw@redhat.com>
Acked-by: Ben Pfaff <blp@ovn.org>
Signed-off-by: Han Zhou <hzhou@ovn.org>

Documentation/conf.py

@@ -65,8 +65,8 @@
             release = line.split(',')[1].strip(string.whitespace + '[]')
             break
 if release is None:
-    sys.stderr.write('%s: failed to determine Open Virtual Network (OVN) version\n'
-                     % filename)
+    sys.stderr.write('%s: failed to determine Open Virtual Network (OVN) '
+                     'version\n' % filename)
     sys.exit(1)
 
 # The short X.Y version.