Update wording in install-a-demo.md

[rosiel]

Purpose / why

I was confused by the installation instructions, and I find this wording clearer.

What changes were made?

• minor wording adjustments to capitalize the Starter Site, clarify overlap of content, use the word "demo" consistently and as a common noun
• Included a step missing from the docs for first-time Portainer users
• Clarify (what I think is) the order of operations.

Verification

Interested Parties

• @Islandora/documentation
• @Islandora/committers

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?

[ysuarez]

If I have time I will uninstall Portianer plugin when I test this.

[ysuarez]

This is an improvement of the curent docs. This should be merged.

[ysuarez]

I think this updated line is an improvement, but my first thought is that we should make it a bit more clear that they have to download a ".zip" file. Since the Google Drive UI puts the actual download button on the upper right, and may not be clear to some folks how to actually download the ".zip" file since I am not sure you can try to download only the ".yml" file via Google Drive.

[rosiel]

Agreed!