docs: fix two typos

[vaibhav8a]

Two typo fixes.

mkdocs/docs/quickstart.md:52 — the sample dstack apply output shows a fleet named defalut:

  FLEET    INSTANCE  BACKEND  RESOURCES  PRICE  STATUS  CREATED
  defalut  -         -        -          -      -       10:36

This is a transcription typo in the pasted output rather than a real name: defalut occurs exactly once in the whole repository — this line — and nothing in src/ ever names a fleet that. Corrected to default so the docs match what the CLI actually prints.

mkdocs/blog/posts/kubernetes-beta.md:127 — "many [diffrent options]" → "different".

Docs only; the column alignment in the code block is unchanged since defalut and default are the same length.