Updated fragment shader api to reflect new uniform-by-name accessors.

[gaaclarke]

DO NOT LAND UNTIL flutter/flutter#176417 IS ON STABLE

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

Updated documentation to take advantage of new easier api.

Issues fixed by this PR (if any):

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

flutter/flutter#176417

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.

[flutter-website-bot]

Visit the preview URL for this PR (updated for commit 0174d55):

https://flutter-docs-prod--pr12556-uniform-by-name-ol1x2zt1.web.app

[sfshaza2]

lgtm