[rb] fix incorrect method name

Author: titusfortner

rb/lib/selenium/webdriver/common/takes_screenshot.rb

@@ -49,7 +49,7 @@ def save_screenshot(png_path, full_page: false)
       # @api public
 
       def screenshot_as(format, full_page: false)
-        if full_page && !respond_to(:full_page)
+        if full_page && !respond_to?(:full_page)
           raise Error::UnsupportedOperationError, "Full Page Screenshots are not supported for #{inspect}"
         end