Fix getting started

Hi,

Getting started instructions don't work due to typos:

```
git clone git@github.com/CSCfi/sd-lock-util.git
cd sd-lock-util
pip install -r requirements.txt
pip install .[]
```

First line throws `fatal: repository 'git@github.com/CSCfi/sd-lock-util.git' does not exist`
Last command doesn't work

Change to these:

```
git clone https://github.com/CSCfi/sd-lock-util.git
cd sd-lock-util
pip install -r requirements.txt
pip install .
```

Start by locating the getting-started instructions containing requirements.txt and the shown setup commands. Compare them with the commands in the issue and verify that the documented clone and installation steps work from a clean checkout.

Written by the indexing model from the issue text.