Remove duplicated sentence in README.md

[amd-justchen]

Purpose

Clean up documentation by removing a repeated sentence:
If you place the ephemeral OS disk in the temporary storage disk, the final size of the temporary disk will equal the initial size of the temporary disk size minus the OS image size.

Does this introduce a breaking change?

[ ] Yes
[x] No

Pull Request Type

What kind of change does this Pull Request introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[x] Documentation content changes
[ ] Other... Please describe:

How to Test

N/A

What to Check

N/A

Other Information

N/A