Capitalise references to Mark for consistency

[sauntimo]

Is this adding or improving a feature or fixing a bug?

• feature: improving docs

What's the new behavior?

• Capitalises references to "Mark" in the commands.md docs.
• Currently both mark and Mark are used frequently and which could lead to confusion around whether they are the same thing - at least for me.
• They all link to the same page and it appears they are referring to the same entity.
• In mark.md the capitalised Mark is used so I thought it should be standardised on that.
• Screenshot of current situation
  
• Screenshot with changes
  

How does this change work?

• Trivial docs update for clarity

Have you checked that...?

• The new code matches the existing patterns and styles.
• The tests pass with yarn test.
• The linter passes with yarn lint. (Fix errors with yarn prettier.)
• The relevant examples still work. (Run examples with yarn watch.)

Does this fix any issues or need any specific reviewers?

Fixes: n/a
Reviewers: Any

[ianstormtaylor]

Beautiful, thank you @sauntimo!