Remove endless warning in splash screen docs

[camsim99]

Fixes endless warning in the splash screen docs: https://docs.flutter.dev/development/ui/advanced/splash-screen?tab=android-splash-alignment-kotlin-tab. Also makes minor grammatical changes.

Presubmit checklist

• [x] This PR doesn’t contain automatically generated corrections (Grammarly or similar).
• [x] This PR follows the Google Developer Documentation Style Guidelines — for example, it doesn’t use i.e. or e.g., and it avoids I and we (first person).
• [x] This PR uses semantic line breaks of 80 characters or fewer.