Improvements to Demo instructions
Improvements to Demo instructions

Summary:
Added notes to the instruction about 3.0.0-beta branche and memsql-ciab docker image name

**Docs impact**:

Test Plan: manually

Reviewers: carl, roxanna

Reviewed By: carl

Subscribers: engineering-list

Differential Revision: https://grizzly.internal.memcompute.com/D38767

memsql/memsql-spark-connector