Update more-plain-language-public-sector.md

Adding clarification around prerequisite

docs/pages/content-design/more-plain-language-public-sector.md

@@ -32,6 +32,6 @@ We host the course through CalLearns. Space is limited.
 
 To take the course, you need:
 
-* To have taken [Introduction to plain language for the public sector](/content-design/introduction-plain-language-public-sector/)
+* To have taken [Introduction to plain language for the public sector](/content-design/introduction-plain-language-public-sector/) through CalLearns. This can have been with an instructor or the self-paced online course. The ODI-hosted webinar alone is not enough.
 * A CalLearns account to register for the training
 * Approval from your manager