Fix: requirements.txt was not written to the disk

While the Python packages requirements are correctly detected and saved
into the json task file, requirements.txt was always empty. Since it was
trying to get packages with the wrong key, packages were always empty
when writing in requirements.txt

Author: arnaudgelas

metagpt/actions/project_management.py

@@ -100,7 +100,7 @@ async def _merge(self, system_design_doc, task_doc, schema=CONFIG.prompt_schema)
     @staticmethod
     async def _update_requirements(doc):
         m = json.loads(doc.content)
-        packages = set(m.get("Required Python third-party packages", set()))
+        packages = set(m.get("Required Python packages", set()))
         file_repo = CONFIG.git_repo.new_file_repository()
         requirement_doc = await file_repo.get(filename=PACKAGE_REQUIREMENTS_FILENAME)
         if not requirement_doc: