Clarify revert before "Add a custom module" in workshop

[chrismayer]

There is one thing that would deserve to be fixed in the workshop as we finish the "Use a module" part with a modified theme (pink borders and footer texts changed) then do the "Custom functionality" part as if we didn't make those changes and didn't add the layers and zoom-to-max-extent modules to eventually get to the "Add a custom module" part which resumes as if "Custom functionality" didn't exist...
I tried to find how changes could be minimized at that time and think we could address this by taking new screenshots for the "Custom functionality" part and change the beginning text of "Add a custom module" with something like

So far we modified the functionality of Wegue using the configuration file or by writing code into WguApp.vue. Now we will create a custom module using JavaScript. First we will revert what was made in WguApp.vue in the previous section. Then, to effectively create the custom module, we need to change a couple of files:

Based on a comment in #389 by @sronveaux