Fix and improve code samples

[alexandersandberg]

Description of what this PR is changing or adding, and why:

• Fixed "Styling buttons" SwiftUI code sample
• Fixed inconsistent indentation in code samples

Issues fixed by this PR (if any):

PRs or commits this PR depends on (if any):

Presubmit checklist

• If you are unwilling, or unable, to sign the CLA, even for a tiny, one-word PR, please file an issue instead of a PR.
• If this PR is not meant to land until a future stable release, mark it as draft with an explanation.
• 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 pronouns).
• This PR uses semantic line breaks
  of 80 characters or fewer.

[gemini-code-assist]

Code Review

This pull request standardizes the indentation of SwiftUI code snippets in the documentation to a 2-space format and corrects a syntax error in a button example. A review comment suggests adding a .padding() modifier to the button styling example to ensure parity with the corresponding Flutter example and recommends indenting view modifiers for consistency with other sections.