docs: fix fork clone command in contributing guide

[chenlichao]

Summary

Fix the fork-cloning command in CONTRIBUTING.md to use the current repository name, cli.git, and explicitly name the checkout directory httpie so the following cd httpie command and later paths remain correct.

The guide links to httpie/cli, but the clone command still assumes a fork named httpie. A newly created fork with GitHub's default name is <YOU>/cli, so the documented remote does not exist for that fork.

This is intentionally a one-line, documentation-only change (1 addition, 1 deletion); no runtime behavior changes.

Verification

• Created a default-named fork of httpie/cli and confirmed its parent and name through the GitHub API.
• Tested the documented remote with git ls-remote: the old chenlichao/httpie.git target failed with “Repository not found”; the corrected chenlichao/cli.git target resolved to the same HEAD as upstream master. Used Git's per-command SSH-to-HTTPS URL rewrite for these checks because authentication is configured over HTTPS.
• Extracted the updated Getting the code shell commands, substituted the account name, and checked them with bash -n.
• Confirmed the explicit clone destination matches the subsequent cd httpie.
• git diff --check passed; manually reviewed the one-line diff.

The application test suite was not run because only a Git command in the contributor guide changes. No full clone or SSH authentication test was performed.