Small changes to API guide and images API

[rfrenchseti]

Fix typos in api guide. Make missing image files show up as blank fields in images api.

• Fixes #N/A
• Were any Django, import pipeline, table_schema, or dictionary files modified? Y
  • Database used: opus3_test_200517
  • All Django tests pass: Y
• Were any JavaScript or CSS files modified? N
  • JSHINT run on all affected files: NA
  • Tested on Chrome, Firefox, Safari, and iOS: NA (Remember to test all browser sizes)
  • Tested for race conditions (with delayed API calls if applicable): NA

Description of changes:

Known problems: