docs: fix topic-branch push instructions in CONTRIBUTING

[shuofengzhang]

What changed

• Fixed a grammar typo in CONTRIBUTING.md ("This is where main development takes place").
• Corrected the topic-branch push example to match the documented workflow:
  • Updated wording from pushing to master to pushing to the topic branch.
  • Updated command example to git push origin bug-123-short-issue-description.

Why

• The guide currently describes topic-branch development, but one command example still points contributors to push master.
• This can confuse first-time contributors and lead to pushing the wrong branch.

Testing

• scripts/clone_and_test.sh QuantConnect/Lean
• Tests: not run (no tests found)

[Martin-Molinero]

Thanks!