[slides][model] Harmonize dumux style

slides/model.md

 ---
-title: Implementing a Model in DuMuX
+title: Implementing a Model in DuMu^x^
 ---
 
-# What is a DuMu<sup>X</sup> model
+# What is a DuMu^x^ model
 
-## What is a DuMu<sup>X</sup> model
-A DuMu<sup>X</sup> model is an implementation of a discretized **mathematical model**, generally given by partial differential equations.
+## What is a DuMu^x^ model
+A DuMu^x^ model is an implementation of a discretized **mathematical model**, generally given by partial differential equations.
 
-## What is a DuMu<sup>X</sup> model
+## What is a DuMu^x^ model
 Mathematical model (PDE):
 $\begin{equation*}
 \small
@@ -22,7 +22,7 @@ $\begin{equation*}
 |B| \frac{S_h(\mathbf{u}^{n+1}_h) - S_h(\mathbf{u}^{n}_h)}{\Delta t}  + \sum_{\sigma \in \Sigma_B} F_{B,\sigma}(\mathbf{u}^{n+1}_h) = \int_{B}  q^{n+1} \, dx, \quad \forall t_{n+1}\leq T, \; \forall B
 \end{equation*}$
 
-## What is a DuMu<sup>X</sup> model
+## What is a DuMu^x^ model
 Discrete model, e.g. using finite volumes:
 $\begin{equation*}
 \small
@@ -33,7 +33,7 @@ $\begin{equation*}
 * $F_{B,\sigma}:$ flux term over sub control volume face (scvf)
 * $q:$ source term
 
-Where to implement these terms in DuMu<sup>X</sup>?
+Where to implement these terms in DuMu^x^?
 
 `LocalResidual`