Clarify explanation of staging area

[alex-ball]

The explanation of the staging area in episode 2 was reproduced from the Software Carpentry Git Novice lesson. The original wording relied on people reading "snapshot" in just the photographic sense, and understanding "the extra" in sense of an archetype ("you know, that guy"). But people may also be familiar with other senses of "snapshot", and I for one thought that an earlier introduction to this extra had been accidentally deleted.

The text has since been clarified in the Software Carpentry lesson. This PR reproduces edits made in swcarpentry/git-novice@e6fe3e6 from PR swcarpentry/git-novice#707 (credit to @vyasr).

[github-actions]

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🔍 Inspect the changes: https://github.com/LibraryCarpentry/lc-git/compare/md-outputs..md-outputs-PR-189

The following changes were observed in the rendered markdown documents:

 02-getting-started.md | 8 ++++----
 md5sum.txt            | 2 +-
 2 files changed, 5 insertions(+), 5 deletions(-)

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⏱️ Updated at 2025-08-28 11:21:37 +0000

[srerickson]

lgtm!