update ONBOARDING.md for meeting template changes (#702)

Typically new TSC members will have been invited as observers to TSC
meetings prior to being onboarded. Update the onboarding instructions
to also update the list of observers meeting template.

Author: richardlau

ONBOARDING.md

@@ -16,7 +16,7 @@ invalid and are superseded by the TSC Charter.
 * Add them to the `@nodejs-private/security-tsc` team.
 * Add them to the `tsc` and `crypto-export` mailing lists.
 * Update the `@nodejs/node` repository README to reflect membership in the TSC.
-* Update https://github.com/nodejs/create-node-meeting-artifacts/blob/master/templates/invited_tsc to reflect membership in the TSC.
+* Update https://github.com/nodejs/create-node-meeting-artifacts/blob/master/templates/invited_tsc and https://github.com/nodejs/create-node-meeting-artifacts/blob/master/templates/observers_tsc to reflect membership in the TSC.
 
 ## Offboarding