Revise screenshot size guidelines in exercise template

Updated screenshot size recommendation from 800x600px to 1200x900px.

Author: GraemeMalcolm

Instructions/Labs/01-exercise-template.md

@@ -52,7 +52,7 @@ First, you need to ...
     # indented code block.
     ```
 
-1. If you need to include a acreenshot, resize it to an appropriate size (so any "normal" formatted text in a partial screenshot is roughly the same size as this text - generally try to make screenshots of full application windows 800x600px (approx)). Store images in a **Media** subfolder and use markdown to add it to the page (remembering that file and folder names are case-sensitive). If the image is in a list, indent it, like this:
+1. If you need to include a acreenshot, resize it to an appropriate size (so any "normal" formatted text in a partial screenshot is roughly the same size as this text - generally try to make screenshots of full application windows 1200x900px (approx)). Store images in a **Media** subfolder and use markdown to add it to the page (remembering that file and folder names are case-sensitive). If the image is in a list, indent it, like this:
 
     ![A screenshot of an application.](./Media/edge-copilot.png)