docs: update quickstart.md

Description of your changes: In quickstart.md, git clones the repo by default in the nfs directory, not root.

Which issue is resolved by this Pull Request: Resolves #44

Checklist:

[ ] Commit Message Formatting: Commit titles and messages follow guidelines in the developer guide.
[ ] Skip Tests for Docs: Add the flag for skipping the build if this is only a documentation change. See here for the flag.
[ ] Skip Unrelated Tests: Add a flag to run tests for a specific storage provider. See test options.
[ ] Reviewed the developer guide on Submitting a Pull Request
[ ] Documentation has been updated, if necessary.
[ ] Unit tests have been added, if necessary.
[ ] Integration tests have been added, if necessary.
[ ] Pending release notes updated with breaking and/or notable changes, if necessary.
[ ] Upgrade from previous release is tested and upgrade user guide is updated, if necessary.
[ ] Code generation (make codegen) has been run to update object specifications, if necessary.

rook / nfs