Update /target_branch slash command description to be more consistent

app/services/slash_commands/interpret_service.rb

@@ -410,7 +410,7 @@ module SlashCommands
     params '@user'
     command :cc
-    desc 'Define target branch for MR'
+    desc 'Set target branch'
     explanation do |branch_name|
       "Sets target branch to #{branch_name}."
     end

changelogs/unreleased/dm-target-branch-slash-command-desc.yml

+---
+title: Update /target_branch slash command description to be more consistent
+merge_request:
+author: