Effective March 2017 Crossref, a major DOI registration organization, recommends changing the way DOIs are displayed in manuscripts and other publications.  DOIs, or Digital Object Identifiers, are unique alphanumeric links used to identify specific content.  Most contemporary peer-reviewed journal articles have a DOI.  DOIs are also used in many citation styles, like Chicago and APA.  The recommended changes include dropping the “dx” from the domain name portion of DOI links and switching to https.  Here is an example:

Old recommended prefix: http://dx.doi.org/

New recommended prefix: https://doi.org/

More information can be found in this article from Crossref: Display guidelines for Crossref DOIs – effective March 2017

APA has adopted the change: DOI Display Guidelines Update (March 2017)

Not sure which display format to use?  Check with your professor for her preference, both the old and new DOI links will be functional indefinitely.