Snake tutorial grammar and naming consistency

[awulkan]

I tried to tidy up the snake tutorial while I was reading it.

Changes:

• Keep capitalization in bullet points consistent (capitalize first letter).
• Keep the spelling of 'tile map' and 'tile source' consistent with their respective documentation page ('tile map' instead of 'tilemap').
• Removed sentence about the FPS being locked to 60.
• Change the word 'pop' to 'remove' to describe using table.remove(). The word 'pop' is often used describe removing the last item of a list in various programming languages, so it could be confusing.
• Various additions and removals of words like 'the'.
• Reworded some sentences to flow better.