Fix UI description in volume snapshot class

Created by: Bettygogo2021

Signed-off-by: Bettygogo2021 91529409+Bettygogo2021@users.noreply.github.com What type of PR is this? /kind documentation

What this PR does / why we need it: Fix a grammar issue in the description.

Special notes for reviewers: /assign @harrisonliu5

Please take a look. Thanks!

Does this PR introduce a user-facing change?

None