Fix expected callers in API Component Overview

[kezike]

After chatting with @dlongley, I confirmed that we are specifying the wrong expected callers in the API Component Overview (currently section 3.7). These are the sections that appear to have issues (unsure about the third one though):

• 3.7.2 Issuer Service: coordinators should be replaced by workflow service
• 3.7.4 Verification Service: coordinators should be replaced by workflow service
• [UNSURE ABOUT THIS] 3.7.5 Holder Service: coordinators should be replaced by workflow service

[msporny]

I think PR #525 might address this issue?