langchain: pubmed tool path update in doc (#16716)
- **Description:** The current pubmed tool documentation is referencing
the path to langchain core not the path to the tool in community. The
old tool redirects anyways, but for efficiency of using the more direct
path, just adding this documentation so it references the new path
- **Issue:** doesn't fix an issue
- **Dependencies:** no dependencies
- **Twitter handle:** rooftopzen
