Update acroform.js

Updated error text document.initForms() to document.initForm() as correct method name is initForm()

lib/mixins/acroform.js

@@ -181,7 +181,7 @@ export default {
   _fieldDict(name, type, options = {}) {
     if (!this._acroform) {
       throw new Error(
-        'Call document.initForms() method before adding form elements to document'
+        'Call document.initForm() method before adding form elements to document'
       );
     }
     let opts = Object.assign({}, options);