Plugin Directory: Typos in ::instance() docblocks

[SergeyBiryukov]

Status_Transitions and Author_Cards classes have typos in ::instance() docblocks:

• Fetch the instance of the Status Transitions class — should be Status_Transitions.
• Fetch the instance of the Author_Card class — should be Author_Cards.

[coffee2code]

In 5313:

Plugin Directory: Fix typos in two ::instance() docblocks.

Props SergeyBiryukov.
Fixes #2714.