update backportrc (#585)

Author: jordan-smith721

.backportrc.json

@@ -4,7 +4,7 @@
   "repoName": "docs-node",
 
   // the branches available to backport to
-  "targetBranchChoices": ["master", "v5.0", "v4.14", "v4.13", "v4.12", "v4.11", "v4.10", "v4.9", "v4.8", "v4.7", "v4.6", "v4.5", "v4.4", "v4.3", "v4.2", "v4.1", "v4.0", "v3.7", "v3.6"],
+  "targetBranchChoices": ["master", "v5.1", "v5.0", "v4.14", "v4.13", "v4.12", "v4.11", "v4.10", "v4.9", "v4.8", "v4.7", "v4.6", "v4.5", "v4.4", "v4.3", "v4.2", "v4.1", "v4.0", "v3.7", "v3.6"],
 
   // Optional: automatically merge backport PR
   "autoMerge": true,