docs(form-checkbox): fix template end tag in "Indeterminate checkbox use-case example" (#6673)
docs(form-checkbox): fix template end tag in "Indeterminate checkbox use-case example" (#6673)

The second template end tag of b-form-group in "Indeterminate checkbox
use-case example" was misplaced.

Co-authored-by: Illya Klymov <7aa1617702f75294d141b42597bffbcb955e2abe@xanf.me>
bootstrap-vue/bootstrap-vue,bootstrap-vue/bootstrap-vue