Update Markdown files

[ErikSchierboom]

We've defined a specification for Markdown files to be applied Exercism-wide. This standard includes, amongst others, the following two rules:

• All files must start start with a level-1 heading (# Some heading text)
• No heading may decend a level greater than one below the previous (e.g. ## may only be followed by ###, not ####)

The Markdown documents of each track should be updated accordingly.