Update textbook.en.md

00.brainstorming-pedagogical-teams/45.synthesis-structuring/electromagnetism/n4/textbook.en.md

@@ -20,7 +20,7 @@ This tree structure may evolve, but it is less agile than the educational conten
 themselves. In themselves, the titles of the chapters and sub-chapters are less important.
 They can be changed easily, they are not the structure.
 
-#### Advice on how to structure this level
+#### Advice on how to structure this level 4
 
 These are **our lectures, restructured and redesigned together**.