[MINOR] Improve the writing of RFC-66

[linliu-code]

Change Logs

Re-structed some sentences, and did not change the original intention.
We do need a new abstract to summary the work proposed in rfc-66.

Impact

Clean the logic of RFC-66.

Risk level (write none, low medium or high below)

None.

Documentation Update

Describe any necessary documentation update if there is any new feature, config, or user-facing change

• The config description must be updated if new configs are added or the default value of the configs are changed
• Any new feature or user-facing change requires updating the Hudi website. Please create a Jira ticket, attach the
  ticket number here and follow the instruction to make
  changes to the website.

Contributor's checklist

• Read through contributor's guide
• Change Logs and Impact were stated clearly
• Adequate tests were added if applicable
• CI passed

[linliu-code]

@danny0405

[hudi-bot]

CI report:

• d953c66 Azure: PENDING

Bot commands

@hudi-bot supports the following commands:

• @hudi-bot run azure re-run the last Azure build