Proper syntax highlight in notes

[jstoobysmith]

HepLean has the ability to write notes using Lean. For example, the following note is in progress:

https://heplean.github.io/HepLean/PerturbationTheory

The Lean code in these notes does not have the correct syntax highlighting for Lean. Fixing it will correspond to modifying the file:

https://github.com/HEPLean/HepLean/blob/master/HepLean/Meta/Notes/ToHTML.lean

which is used to generate the notes.

Any help with this would be great.