Typo corrections in installation docs

[dmlls]

What does this PR do?

Summary

This PR fixes a couple minor typos at https://textattack.readthedocs.io/en/master/0_get_started/installation.html.

Additions

N/A

Changes

Typos have been corrected.

Deletions

N/A

Checklist

• The title of your pull request should be a summary of its contribution.
• Please write detailed description of what parts have been newly added and what parts have been modified. Please also explain why certain changes were made.
• If your pull request addresses an issue, please mention the issue number in the pull request description to make sure they are linked (and people consulting the issue know you are working on it)
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• Make sure existing tests pass.
• Add relevant tests. No quality testing = no merge.
• All public methods must have informative docstrings that work nicely with sphinx. For new modules/files, please add/modify the appropriate .rst file in TextAttack/docs/apidoc.'