Update docs/src/multi-physics_coupling.md

Co-authored-by: Michael Schlottke-Lakemper <[email protected]>

docs/src/multi-physics_coupling.md

@@ -22,7 +22,7 @@ system B with $n$ variables $u_{B,j}$.
 We then define two coupling functions, one that transforms $u_A$ into $u_B$
 and one that goes the other way.
 
-In their minimal form they take the position vector `x` and state vector `u`
+In their minimal form they take the position vector $x$ and state vector $u$
 and return the transformed variables.
 Examples can be seen in `examples/structured_2d_dgsem/elixir_advection_coupled.jl`.