[6.x] Tweak cookie() and plainCookie() return types (#941)

* Fix return types

* Update InteractsWithCookies.php

Co-authored-by: Steve Bauman <[email protected]>
Co-authored-by: Taylor Otwell <[email protected]>

Author: stevebauman

src/Concerns/InteractsWithCookies.php

@@ -16,7 +16,7 @@ trait InteractsWithCookies
      * @param  string|null  $value
      * @param  int|DateTimeInterface|null  $expiry
      * @param  array  $options
-     * @return string
+     * @return $this|string
      */
     public function cookie($name, $value = null, $expiry = null, array $options = [])
     {
@@ -46,7 +46,7 @@ public function cookie($name, $value = null, $expiry = null, array $options = []
      * @param  string|null  $value
      * @param  int|DateTimeInterface|null  $expiry
      * @param  array  $options
-     * @return string
+     * @return $this|string
      */
     public function plainCookie($name, $value = null, $expiry = null, array $options = [])
     {