Remove wrong option in configuration file reference

[DSiekmeier]

There is a wrong statement in the documentation about the display_format format, which is removed in this commit.

Checklist

• I have read the contributing doc.
• I have included a link to the relevant issue number.
  Documentation of "display_format" incorrectly states that it can be used to change date display format  #1617
• I have checked to ensure there aren't other open pull requests
  for the same issue.
• I have written new tests for these changes, as needed.

[micahellison]

Thanks!