[in_app_purchase] Correct mistake with markdown in README.md

[mvanbeusekom]

Removed the code block start tag which broke the formatting of the README markdown.

Fixes flutter/flutter#87185

If you had to change anything in the flutter/tests repo, include a link to the migration guide as per the breaking change policy.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter. (Note that unlike the flutter/flutter repo, the flutter/plugins repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy.
  • Not necessary to release a separate version for this small documentation change.
• I updated CHANGELOG.md to add a description of the change.
  • Not necessary to release a separate version for this small documentation change.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test exempt.
  • Small documentation fix which doesn't require unit-tests.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[BeMacized]

LGTM!