README.activity-report: Clarify “empty pages”.

* README.activity-report: Clarify.

README.activity-report

@@ -104,7 +104,7 @@ _Note_: See between the lines 249 and 256; guess 251?
 *** TODO Generate the two PDFs: =guix build -f doc/build.scm=
 *** TODOs [0/5] Double-check the PDFs
 **** TODO page numbering
-**** TODO empty blank pages
+**** TODO empty blank pages: the booklet must properly render, with cover and back-cover appearing on the same A4 sheet
 **** TODO author list
 **** TODO editor name
 **** TODO date