NIFI-9445: Minor formatting improvements in developer guide.
NIFI-9445: Minor formatting improvements in developer guide.

This closes #5575

Signed-off-by: David Handermann <eb72b581ef337dcba9988f4e8f9ce3c59afe7a28@apache.org>

MikeThomsen/nifi,jfrazee/nifi,jfrazee/nifi,jfrazee/nifi,pvillard31/nifi,MikeThomsen/nifi,MikeThomsen/nifi,pvillard31/nifi,pvillard31/nifi,MikeThomsen/nifi,jfrazee/nifi,pvillard31/nifi,jfrazee/nifi,MikeThomsen/nifi,pvillard31/nifi,jfrazee/nifi,pvillard31/nifi,jfrazee/nifi,MikeThomsen/nifi,MikeThomsen/nifi,pvillard31/nifi,jfrazee/nifi,pvillard31/nifi