Fix `add httpx` example with real git branch (#8756)
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## Summary
The example in the docs for adding a git source with `--branch` fails
because `main` doesn't exist.
```sh
uv add git+https://github.com/encode/httpx --branch main
# error: Git operation failed
# Caused by: failed to fetch into: /Users/manzt/.cache/uv/git-v0/db/4c0b1441d92956e1
# Caused by: failed to fetch branch `main`
# Caused by: process didn't exit successfully: `/usr/bin/git fetch --force --update-head-ok 'https://github.com/encode/httpx' '+refs/heads/main:refs/remotes/origin/main'` (exit status: 128)
```
This PR changes the example to the default branch for httpx, `master`.
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## Test Plan
N/A
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