fix(store): fix typo in runtime checks injection token description

[jtcrowson]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/ngrx/platform/blob/master/CONTRIBUTING.md#commit

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[x] Other... Please describe: Fix spelling in token description.  Was not causing functional issues.

What is the current behavior?

Closes #1972

What is the new behavior?

Fixes typo in _ACTIVE_RUNTIME_CHECKS injection token description.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[ngrxbot]

Preview docs changes for 1f04c70 at https://previews.ngrx.io/pr1975-1f04c70/