docs: sort inputs alphabetically

Currently the inputs are ordered alphabetically (I know underscores
aren't actually part of the alphabet). So, let's maintain this ordering.

action.yml

@@ -116,16 +116,16 @@ inputs:
       Please refer to this action's README for all available placeholders.
     default: >-
       [Backport ${target_branch}] ${pull_title}
+  source_pr_number:
+    description: >
+      Specifty the pull request number to backport instead of detecting based on the pull request closed event.
+      Only valid when github.event_type == "workflow_dispatch".
   target_branches:
     description: >
       The action will backport the pull request to each specified target branch (space-delimited).
       Note that the pull request's headref is excluded automatically.
       Can be used in addition to backport labels.
       By default, only backport labels are used to specify the target branches.
-  source_pr_number:
-    description: >
-      Specifty the pull request number to backport instead of detecting based on the pull request closed event.
-      Only valid when github.event_type == "workflow_dispatch".
 
 outputs:
   created_pull_numbers: