DOC: Small update to velocity distro docstrings.
Added a note on how to specify a random seed in the Maxwell Boltzmann distribution; cleaned up some typos while I was at it.
Checklist
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I am familiar with ASE's contribution guidelines. -
Doc strings in code changed in this MR are up to date. - [NA] Unit tests have been added for new or changed code.
- [NA] Issue is resolved via "closes #XXXX" if applicable.