Clarify that package_json_file is relative to GITHUB_WORKSPACE (#184)

* Clarify that package_json_file is relative to GITHUB_WORKSPACE

Clarify the description for package_json_file parameter to specify that the path must be relative to the repository root.

* Apply suggestion from @zkochan

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Author: github-actions[bot]

action.yml

@@ -16,7 +16,7 @@ inputs:
     required: false
     default: 'null'
   package_json_file:
-    description: File path to the package.json to read "packageManager" configuration
+    description: File path to the package.json to read "packageManager" configuration. This path must be relative to the repository root (GITHUB_WORKSPACE).
     required: false
     default: 'package.json'
   standalone: