Fix installation instructions
Fix installation instructions

Quick start instructions create the file `.shell_prompt.sh`, but the fish instructions use `.promptline.sh`. Seeing as the project is called "promptline", I think that the file should always be called `promptline.sh`.
Otherwise, if you think the file should be called `.shell_prompt.sh`, change the `config.fish` instructions to use it rather than `.promptline.sh`.
edkolev/promptline.vim