[vs code extension] Change the text of "Show OpenAPI3 Documentation" to "Preview API Documentation"

[josefree]

Change the text in shortcut menu and in command palette. The original text may be misunderstood as emitting OpenAPI3. And use "preview" to clarify the documentation generated is not for publishing.

[josefree]

@RodgeFu Please also update the text from "Import TypeSpec from OpenApi3" to "Import TypeSpec from OpenAPI 3". Both context menu and command.