Update Markdown resources and language on the plain text and file formats episode

[morskyjezek]

The episode that discusses markdown (https://librarycarpentry.org/lc-overview/06-file-naming-formatting/index.html) could use a revision for clarity and tone. But more pressing, I think, are that the tools for working with markdown seem confusing. It would make more sense to suggest Dillinger.io as the tool to use (then use it in the exercise), then after that note additional resources that might be useful for working with markdown text.