cli fix: remove unnecessary await from startPreview call

This PR resolves a SonarCloud code quality issue as part of the effort to fix SonarCloud issues reported in https://github.com/asyncapi/cli/issues/1881.

Description

Removed unnecessary await keyword from startPreview call in the preview command
The startPreview function returns void, not a Promise, so awaiting it was incorrect and triggered a linter error: "Unexpected await of a non-Promise (non-"Thenable") value"
This change aligns the preview command with the pattern used in the studio command, where startStudio (also returning void) is called without await on line 58 of studio.ts

Testing

The change is minimal and syntactically correct:

Removed await from line 40 in src/apps/cli/commands/start/preview.ts
The function signature in Preview.ts confirms startPreview returns void
The change matches the existing pattern in studio.ts where startStudio is called without await
No functional changes - the preview command behavior remains the same, only the linter error is resolved

Documentation

No documentation updates required:

The documentation in docs/usage.md is auto-generated from the command definitions
This is an internal code fix that doesn't change the user-facing API or behavior
The command usage, flags, and descriptions remain unchanged

Related issue(s)

Fixes https://github.com/asyncapi/cli/issues/1881