Fix quickstart example to use Megaparsec parse function correctly

[Copilot]

The quickstart example was calling parseGram source directly, but parseGram is a Parser Gram type, not a function that takes a string. Running a Megaparsec parser requires the parse function.

Changes

• Added parse to the import from Text.Megaparsec
• Fixed usage to parse parseGram "" source instead of parseGram source

Before

import Text.Megaparsec (errorBundlePretty)
...
case parseGram source of

After

import Text.Megaparsec (parse, errorBundlePretty)
...
case parse parseGram "" source of

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