Update your_first_project.rst

[cyhunblr]

changed "manipulator" with "panda_arm". This can make other documents easier to understand.

Description

Please explain the changes you made, including a reference to the related issue if applicable

Checklist

• [ ] Required by CI: Code is auto formatted using clang-format
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[sea-bass]

Closing this PR as it has to do with Humble vs. Main versions of the documentation. See here for an explanation:

https://github.com/moveit/moveit2_tutorials/pull/965#issuecomment-2463674431

Hope this helps!