DOC: Make usage of "lowercase" and "uppercase" consistent across all docs

[michaelgrund]

Description of proposed changes

In the docs we use lowercase/uppercase, lower-case/upper-case and lower case/upper case. This PR makes all appearances consistent to use lowercase/uppercase.

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.

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 have no strong opinion on which one is correct or better. So assign this PR to @yvonnefroehlich.

If the PR looks good, please don't forget to also update the following lines:

$ ack '[Ll]ower[ -]case'
pygmt/src/project.py
139:        Use lower case. The output will be columns of values corresponding to

$ ack '[Uu]pper[ -]case'
pygmt/helpers/decorators.py
122:                  change in the x coordinates (upper case to use projected
125:                  change in the y coordinates (upper case to use projected
128:                  distance between coordinates (upper case to use projected

[yvonnefroehlich]

I personally do not really like the hyphen version and the internet says with white space is British and without American, so I agree with @michaelgrund to use the version without white space. Happy that this is now consistent.