Fix mill command in doc

Co-authored-by: Kien Dang <kiend@pm.me>

docs/pages/dev-from-sources.md

@@ -205,7 +205,7 @@ are left (look for `213` to spot 2.13-specific projects).
 It is recommended to [manually generate IntelliJ configuration files](https://com-lihaoyi.github.io/mill/mill/Intro_to_Mill.html#_intellij_support_legacy),
 rather than letting IntelliJ load the project itself. In order to do that, run
 ```text
-$ ./mill.scalalib.GenIdea/idea
+$ ./mill mill.scalalib.GenIdea/idea
 ```
 
 You can then open the project in IntelliJ.