docs: make type: oci description match code

[mikemccracken]

What type of PR is this?
documentation

Which issue does this PR fix:

the description of the type: oci field in from did not match what the code requires

What does this PR do / Why do we need it:

it fixes the description

If an issue # is not available please add repro steps and logs showing the issue:

https://stackerbuild.io/v0.40.1/reference/stacker_file/#from

Testing done on this change:

yolo

Automation added to e2e:

yolo

Will this break upgrades or downgrades?

no

Does this PR introduce any user-facing change?:

docs only

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

Merging #532 (2e9391f) into main (d0b712e) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #532   +/-   ##
=======================================
  Coverage   13.32%   13.32%           
=======================================
  Files          40       40           
  Lines        5863     5863           
=======================================
  Hits          781      781           
  Misses       4954     4954           
  Partials      128      128           

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