fix: minor spelling fix in docs

[Perfect5th]

Fixes Or Enhances

Single-line change fixing a spelling error at the end of the docs.

Make sure that you've checked the boxes below before you submit PR:

• Tests exist or have been written that cover this particular change.

@go-playground/validator-maintainers

[coveralls]

coverage: 73.73%. remained the same
when pulling 6c21dfc on Perfect5th:s-existant-existent
into bdc3a7d on go-playground:master.