fix(requirements.txt): Set requirements path to posix expression so that it can work in windows OS as well. (#3101)

When I cloned langgraph example, I was not able to run the code because
of the requirements.txt file. The path was set to posix expression which
was not working in windows OS.
I have updated the path to posix expression so that it can work in
windows OS as well.

Try running `langgraph-example` in windows using the langgraph-cli in
windows OS. It was working for linux not in windows.

libs/cli/langgraph_cli/config.py

@@ -330,7 +330,7 @@ def _assemble_local_deps(config_path: pathlib.Path, config: Config) -> LocalDeps
                 rfile = resolved / "requirements.txt"
                 pip_reqs.append(
                     (
-                        rfile.relative_to(config_path.parent),
+                        rfile.relative_to(config_path.parent).as_posix(),
                         f"{container_path}/requirements.txt",
                     )
                 )