Format code blocks to make it render (#2553)

Currently the julia code is not being rendered correctly. Hopefully this fixes it.
https://trixi-framework.github.io/TrixiDocumentation/stable/heterogeneous/#Example

Author: cwittens

docs/src/heterogeneous.md

@@ -129,35 +129,35 @@ end
     This will, e.g., work with `u_ode`. Internally, KernelAbstractions.jl's `get_backend`
     will be called, i.e., KernelAbstractions.jl has to know the type of `x`.
 
-    ```julia
-    backend = trixi_backend(u_ode)
-    ```
+ ```julia
+ backend = trixi_backend(u_ode)
+ ```
 
 3.  Add a new argument `backend` to `trixi_rhs_fct` used for dispatch.
     When `backend` is `nothing`, the legacy implementation should be used:
-    ```julia
-    function trixi_rhs_fct(backend::Nothing, mesh, equations, solver, cache, args)
-        @unpack unpacked_args = cache
-        @threaded for element in eachelement(solver, cache)
-            rhs_fct_per_element(element, unpacked_args, args)
-        end
-    end
-    ```
+ ```julia
+ function trixi_rhs_fct(backend::Nothing, mesh, equations, solver, cache, args)
+     @unpack unpacked_args = cache
+     @threaded for element in eachelement(solver, cache)
+         rhs_fct_per_element(element, unpacked_args, args)
+     end
+ end
+ ```
 
 4.  When `backend` is a `Backend` (a type defined by KernelAbstractions.jl), write a
     KernelAbstractions.jl kernel:
-    ```julia
-    function trixi_rhs_fct(backend::Backend, mesh, equations, solver, cache, args)
-        nelements(solver, cache) == 0 && return nothing  # return early when there are no elements
-        @unpack unpacked_args = cache
-        kernel! = rhs_fct_kernel!(backend)
-        kernel!(unpacked_args, args,
-                ndrange = nelements(solver, cache))
-        return nothing
-    end
-
-    @kernel function rhs_fct_kernel!(unpacked_args, args)
-        element = @index(Global)
-        rhs_fct_per_element(element, unpacked_args, args)
-    end
-    ```
+ ```julia
+ function trixi_rhs_fct(backend::Backend, mesh, equations, solver, cache, args)
+     nelements(solver, cache) == 0 && return nothing  # return early when there are no elements
+     @unpack unpacked_args = cache
+     kernel! = rhs_fct_kernel!(backend)
+     kernel!(unpacked_args, args,
+             ndrange = nelements(solver, cache))
+     return nothing
+ end
+
+ @kernel function rhs_fct_kernel!(unpacked_args, args)
+     element = @index(Global)
+     rhs_fct_per_element(element, unpacked_args, args)
+ end
+ ```