doc(setting): modify the setting description

[mantissahz]

Which issue(s) this PR fixes:

Issue # longhorn/longhorn#9391

What this PR does / why we need it:

Modify the setting Default Longhorn Static Storage Class Name description.

Special notes for your reviewer:

Additional documentation or context

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[derekbit]

@mantissahz Need to update v1.8.0-dev and v1.6.3-dev as well. Thank you.

[mantissahz]

@mantissahz Need to update v1.8.0-dev and v1.6.3-dev as well. Thank you.

Added. Thanks