Fix the example for editor.cmd (#1313)

Fix the example for editor.cmd (#1313)

Using double quotes, the  ${GPR_FILE} is interpreted as a variable by Bourne-like shells and evaluated to an empty string or whatever value has when executed. Single quotes is a way to avoid that evaluation.

This example will not make Emacs select and load the GPR_FILE as the project file for ada-mode, that would be more cumbersome (and requires ada-mode 8.0.3), but I didn't want to complicate the example:
```
alr config --set --global editor.cmd 'emacs --eval=(ada-build-prompt-select-prj-file"${GPR_FILE}") ${GPR_FILE}'
```