Fix header formatting in guide.md

[0706925]

Description

Type(s)

• Game page addition
• Game page modification
• Other wiki edit

Verification

Have you

• Followed the contribution guidelines?
• Ran ./scripts/generate.mjs?
• Ran ./scripts/lint.mjs?
• Checked spelling and grammar?
• (If applicable) Provided documentation for a game not working? (i.e. detailing what happens when the game doesn't work?)
• (If applicable) Ensured the linked Steam ID is the correct one for your game addition?