kaleidoscope-builder: Improved pre-flash prompt

Based on #306, with slightly improved text. Thanks to Ross Donaldson
(@Gastove) for the original pull request!

Closes #306.

Signed-off-by: Gergely Nagy <algernon@keyboard.io>

bin/kaleidoscope-builder

@@ -70,7 +70,7 @@ prepare_to_flash () {
         size
     fi
 
-    echo "Press ENTER when ready..."
+    echo "When ready, hold down PROG, and press ENTER. Keep holding PROG until flashing starts..."
 
     # We do not want to permit line continuations here. We just want a newline.
     # shellcheck disable=SC2162