Fix MATLAB syntax blocks in 'Testing a function'

[warrickball]

I found two solutions under Testing a function that were being rendered as:

```
function out = normalise(in)
    %NORMALISE   Return original array, normalised so that the
    %            new values lie in the range 0 to 1.

    H = max(max(in));
    L = min(min(in));
    out = (in-L)/(H-L);
end
```
{: .language-matlab}

and

```
a = linspace(1, 10);   % Create evenly-spaced vector
norm_a = normalise(a); % Normalise vector
plot(a, norm_a)        % Visually check normalisation
```
{: .language-matlab}

I suspect these are from the old style (pre-Workbench) lessons, so I mimicked the syntax used elsewhere for MATLABL function blocks, i.e.

```matlab
% code here
```

[github-actions[bot]]

:ok: Pre-flight checks passed :smiley:

This pull request has been checked and contains no modified workflow files, spoofing, or invalid commits.

It should be safe to Approve and Run the workflows that need maintainer approval.