docs(reset): change semi-colon to colon in code example

The first code example for the reset function was invalid as it has a semi-colon instead of a colon for the last property in the json object.  Change the semi-colon to a colon.

Closes https://github.com/angular/angular/issues/12531

modules/@angular/forms/src/model.ts

@@ -983,7 +983,7 @@ export class FormGroup extends AbstractControl {
    * ### Example
    *
    * ```ts
-   * this.form.reset({first: 'name', last; 'last name'});
+   * this.form.reset({first: 'name', last: 'last name'});
    *
    * console.log(this.form.value);  // {first: 'name', last: 'last name'}
    * ```