I see that the Wiki article is messed up due to formatting. Here’s what you’re supposed to do:
In the Edit Sign Form section, the HTML at in the bottom text box has:
==signform== (the equal signs are what messed up the Wiki entry).
Go to your current form and copy all the HTML code in the “form method=post” section between the tags. Copy all that, then paste it to replace ==signform== and fix the Sign the Book button text.