Add missing comma to branch-1.9 in conf.py (#1599)

Author: dgarcia360

docs/source/conf.py

@@ -14,7 +14,7 @@
 # Builds documentation for the following tags and branches.
 TAGS = []
 BRANCHES = [
-    "branch-1.9"
+    "branch-1.9",
     "branch-1.8",
     "branch-1.7",
     "branch-1.6",