Fix a few misspellings

[ysuarez]

Purpose / why

To fix a few simple misspellings by running the aspell CLI spellchecker following these steps...
https://islandora-collaboration-group.github.io/ISLE/contributor-docs/spell-checker/

What changes were made?

Just changed a few misspelled words, and tweaked the glossary definition for Mirador.

Verification

Pleaser review individual to see if I made a mistake.

Interested Parties

@Islandora/documentation

Checklist

Pull-request Reviewer

Pull-request reviewer should ensure the following:

• Does this PR link to related issues?
• Does the proposed documentation align with the Islandora Documentation Style Guide?
• Are the changes accurate, useful, free of typos, etc?

Person Merging

The person merging should ensure the following:

• Does mkdocs still build successfully? (This is indicated by TravisCI passing. To test locally, and see warnings, see How To Build Documentation.)
• If pages are renamed or removed, have all internal links to those pages been fixed?
• If pages are added, have they been linked to or placed in the menu?
• Did the PR receive at least one approval from a committer, and all issues raised have been addressed?