material: Fix instructions for generating screenshots

Author: tronical

ui-libraries/material/docs/README.md

@@ -42,16 +42,11 @@ not be processed and optimized by Starlight.
 
 The docs use a lot of autogenerated content. First create all the screenshots which will be placed at `src/assets/generated/`.
 
-As a one-time install, run
-
-```
-cargo install --git https://github.com/slint-ui/slint slint-docsnapper
-```
-
 Generate the screenshots:
 
 ```bash
-slint-docsnapper docs --overwrite
+cd ui-libraries/material
+cargo run -p slint-docsnapper -- -Lmaterial=$PWD/src/material.slint docs
 ```
 
 Build the docs: