chore(slides): add commented theme-set option in watch-slides-internal

This change provides a reference for an alternative theme configuration,
improving future customization flexibility.

- Added a commented line for the theme-set option in the
  watch-slides-internal target
- Ensures developers have a quick reference for enabling the 'glenux'
  theme without altering the current setup

Signed-off-by: Glenn Y. Rolland <glenux@glenux.net>

tasks/slides.mk

@@ -16,6 +16,7 @@ watch-slides-internal: .marp/theme.css
 		 --theme $$(pwd)/.marp/theme.css \
 		 -w $(SLIDES_DIR) \
 		 -s
+	 # --theme-set glenux
 
 .PHONY: watch-slides serve-slides
 watch-slides: ## run development server for PDF slides