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`.

<!-- What's the purpose of the change? What does it do, and why? -->

## Test Plan

N/A

<!-- How was it tested? -->

docs/concepts/dependencies.md

@@ -140,7 +140,7 @@ A revision (i.e., commit), tag, or branch may also be included:
 
 ```console
 $ uv add git+https://github.com/encode/httpx --tag 0.27.0
-$ uv add git+https://github.com/encode/httpx --branch main
+$ uv add git+https://github.com/encode/httpx --branch master
 $ uv add git+https://github.com/encode/httpx --rev 326b9431c761e1ef1e00b9f760d1f654c8db48c6
 ```