Fix typo: "unfimiliar" → "unfamiliar" in reading-a-codebase.md

[mergeworthy]

The opening paragraph of the "Reading a codebase" guide says unfimiliar instead of unfamiliar.

Where

guides/reading-a-codebase.md, the first sentence:

Before you change a line of code in an unfimiliar codebase, you need to orient yourself.

What it should say

Before you change a line of code in an unfamiliar codebase, you need to orient yourself.

Acceptance criteria

• unfimiliar is replaced with unfamiliar.
• No other text in the file is changed.
• The PR is small and focused.

Estimated time

~5 minutes.

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