Update 04_extensions_to_the_euler_lagrange_equation.tex

[Kaiwen-S]

Fixing the notation for divergence. \div is rendered as $\div$. I changed it to $\nabla\cdot$

[zeramorphic]

Huh, this appears to be a regression. physics is supposed to redefine \div... I'm going to keep the issue open for now to remind myself to do a fix. I think the better solution would be to investigate why this behaviour has stopped happening and to fix it. Thanks for bringing it to my attention!

[zeramorphic]

The issue's been fixed. physics just makes its redefinitions at a weird time, before unicode-math overrides all the symbols itself (I think). I've changed the time at which the redefinitions happen and this has fixed the behaviour.