x/playground: fix vertical alignment of "Imports" checkbox

[smasher164]

When examples were added to the playground, the banner was changed to use flexbox. However, the "Imports" checkbox lost its vertical centering as a result.

This is what is currently looks like:

This is what it should look like:

[gopherbot]

Change https://golang.org/cl/294889 mentions this issue: playground: fix vertical alignment of "Imports" checkbox