gh-104301: Tidy pdb documentation of exclamation prefix

[SnoopJ]

This changeset fixes#104301 by removing an errant space and adding an explicit example of using the prefix for disambiguation of built-in next from the pdb command of the same name

• Issue: Misleading documentation of ! prefix in pdb #104301

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📚 Documentation preview 📚: https://cpython-previews--104302.org.readthedocs.build/

[SnoopJ]

Edit: I attempted to fix the CI failure in a0124a2

Failure in CI appears to be due to the -W flag in the sphinx-build invocation, converting the following warning into a fatal error:

Warning: /home/runner/work/cpython/cpython/Doc/library/pdb.rst:607: WARNING: Could not lex literal_block as "python3". Highlighting skipped. 

I think the code block in question may need an explicit declaration, but someone with better Sphinx skills than me can probably better spot what the fix for that is.

[iritkatriel]

This can be close, right?

[SnoopJ]

Yup, closing in favor of #104342