Add missing spellcheck param to character count

We added a new spellcheck param to the textarea component in #1859

However, we forgot to add the param to the character count component, which meant it couldn't be passed through to the textarea. This adds the missing param.

src/govuk/components/character-count/template.njk

@@ -10,6 +10,7 @@
     name: params.name,
     describedBy: params.id + '-info',
     rows: params.rows,
+    spellcheck: params.spellcheck,
     value: params.value,
     formGroup: params.formGroup,
     classes: 'govuk-js-character-count' + (' govuk-textarea--error' if params.errorMessage) + (' ' + params.classes if params.classes),