openmpi-nightly-tarball.sh: fix typo

(cherry picked from commit 0e21c66)

Author: jsquyres

contrib/build-server/openmpi-nightly-tarball.sh

@@ -29,7 +29,7 @@ if [ $# -eq 0 ] ; then
     # We're no longer ever checking the 1.0 - 1.4 branches anymore
     branches="master v1.8 v1.6"
 else
-    dirs=$@
+    branches=$@
 fi
 
 # Build root - scratch space