correct the sample usage of diff_api.py (#19248)

* correct then sample command of diff_api.py to align with paddle_build.sh

tools/diff_api.py

@@ -30,6 +30,6 @@ if error:
         '''If you modify/add/delete the API files, including code and comment, please follow these steps in order to pass the CI:
     1. cd ${paddle_path}, compile paddle;
     2. pip install build/python/dist/(build whl package);
-    3. run "python tools/print_signatures.py paddle.fluid,paddle.reader > paddle/fluid/API.spec"'''
+    3. run "python tools/print_signatures.py paddle.fluid> paddle/fluid/API.spec"'''
     )
     sys.exit(1)