doc: fix invalid Commands.md example for introspect [skip ci]

It requires at least one option argument, and the one that provides
"basic information" about the project is --projectinfo, so let's use
that to demo the command.

Fixes #8182

docs/markdown/Commands.md

@@ -151,7 +151,7 @@ Displays information about a configured meson project.
 
 Display basic information about a configured project in `builddir`:
 ```
-meson introspect builddir
+meson introspect builddir --projectinfo
 ```
 
 ### install