Mentioning PDEs on the page for ODEs

[hurak]

In docs/src/lecture_13/theory.md somewhere in the middle of the page we read

"The theory of partial differential equations is complicated because they employ a special definition of derivative (weak derivative), and the solution is defined on special spaces (Sobolev spaces)."

Why is it mentioned here? The whole page is focused just on ODEs, this remark about PDEs in the middle of a text on ODEs makes no sense here.