fix: update fields type in update_project_item documentation

Author: nickytonline

README.md

@@ -568,7 +568,7 @@ The following sets of tools are available (all are on by default):
   - `query`: Filter projects by a search query (matches title and description) (string, optional)
 
 - **update_project_item** - Update project item
-  - `fields`: A list of field updates to apply. (array, required)
+  - `fields`: A list of field updates to apply. (object[], required)
   - `item_id`: The numeric ID of the project item to update (not the issue or pull request ID). (number, required)
   - `owner`: If owner_type == user it is the handle for the GitHub user account. If owner_type == org it is the name of the organization. The name is not case sensitive. (string, required)
   - `owner_type`: Owner type (string, required)