Update DebugMenu.md (#2157)

Clarify that the -/B button needs to be held to get the debug menu.

Author: Steltek

Documentation/DebugMenu.md

@@ -3,7 +3,7 @@
 In this firmware there is extra debugging information in a hidden sub-menu.
 This menu is meant to be simple, so it has no fancy GUI animations.
 
-- Access it by pressing the rear button (`-/B`) on the iron while it is on the home screen.
+- Access it by pressing and holding the rear button (`-/B`) on the iron while it is on the home screen.
 - Use the front button (`+/A`) to scroll through the menu.
 - To exit, use the rear button (`-/B`) again.
 
@@ -155,3 +155,4 @@ The mod method is shown in the [February 2022 PINE64 community updates](https://
 :--------
 
 
+