DOC: Change "Pandas" to "pandas"

[yvonnefroehlich]

Description of proposed changes

Change Pandas to pandas to be consistent with the pandas documentation.
For context: #3475 (comment)

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If wrapping a new module, open a 'Wrap new GMT module' issue and submit reasonably-sized PRs.
• If adding new functionality, add an example to docstrings or tutorials.
• Use underscores (not hyphens) in names of Python files and directories.

Slash Commands

You can write slash commands (/command) in the first line of a comment to perform
specific operations. Supported slash command is:

• /format: automatically format and lint the code

[seisman]

I'm OK with the changes. There are still a few files to be updated:

• .github/workflows/ci_tests_dev.yaml
• .github/workflows/ci_tests.yaml
• .github/ISSUE_TEMPLATE/4-release_checklist.md