Update README.md with correct `cd` command

[crosskayla]

nitpick: incorrect and / or misleading cd command

explanation: The slash / at the beginning of the path points to the root of the filesystem, so cd /notion-sdk-js tries to change to a directory that is directly under the root directory. That is not where the cloned repo would be based on the first git clone command

reason for change: could confuse some beginners 🤷‍♀️