[shared_preferences] Fix example in readme for mocking values

[ueman]

This fixes the example for mocking values in the readme of shared_preferences.
After the NNBD migration mock values can only be of type Object.

There are no issues for this small readme error.

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.
• I updated CHANGELOG.md to add a description of the 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.
• All existing and new tests are passing.

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

[stuartmorgan]

Thanks for the contribution! A few small things inline:

[stuartmorgan]

LGTM, thanks! @bparrishMines for second review.

[stuartmorgan]

@bparrishMines Ping on this trivial review.

[bparrishMines]

Just one small nit and it looks like the CHANGELOG needs one more update.

[stuartmorgan]

@bparrishMines It's been updated; PTAL

[bparrishMines]

LGTM

[fluttergithubbot]

This pull request is not suitable for automatic merging in its current state.

• This commit is not mergeable and has conflicts. Please rebase your PR and fix all the conflicts.

[stuartmorgan]

@ueman Could you update this for the version conflict so we can land it? (I'm not sure why the GitHub UI is showing this as not having conflicts, it definitely does.)

[stuartmorgan]

Nevermind, the GitHub UI issue resolved itself, so I was able to resolve the conflicts directly.

[fluttergithubbot]

This pull request is not suitable for automatic merging in its current state.

• The status or check suite publishable has failed. Please fix the issues identified (or deflake) before re-applying this label.

[stuartmorgan]

This needs master merged in to pick up the fix for the publishable failure (which was caused by a pub change). I'm not able to do that from the GitHub UI without a conflict to resolves, so @ueman please merge in master and push when you have a chance.