Correct inconsistencies in neovim-external-editor-setup.md

[Pituivan]

📝 Summary

Just corrected two minor mistakes in doc/neovim-external-editor-setup.md guide.

🔍 Details

1. The guide instructs to create a file named godotdev.sh, but from that moment on, that file is referred to as just godotdev. I simply added .sh extension to all subsequent mentions of the script.
2. Line 232 says to execute ~/.local/bin/godot-nvr.sh /path/to/file.gd:10 to check if the script works standalone, but that's not how you use the script — you don't add :<line number> after the file path, you pass +<line number> as first argument. I corrected this line so it matches the correct usage of the script: ~/.local/bin/godot-nvr.sh +10 /path/to/file.gd.