Fix typo in script.

Change-Id: I853bb5758005f9f7c4940f815c3d72a1d48acd21
diff --git a/tools/buildbot-build.sh b/tools/buildbot-build.sh
index 192ee6d..77e6b1a 100755
--- a/tools/buildbot-build.sh
+++ b/tools/buildbot-build.sh
@@ -52,7 +52,7 @@
     android_root=$1
     shift
   elif [[ "$1" == -j* ]]; then
-    j_arg = $1
+    j_arg=$1
     shift
   elif [[ "$1" == "" ]]; then
     break