[py] fix typing in get_full_page_screenshot_as_png (#11163)

Author: eNcacz

py/selenium/webdriver/firefox/webdriver.py

@@ -336,7 +336,7 @@ def save_full_page_screenshot(self, filename) -> bool:
         """
         return self.get_full_page_screenshot_as_file(filename)
 
-    def get_full_page_screenshot_as_png(self) -> str:
+    def get_full_page_screenshot_as_png(self) -> bytes:
         """
         Gets the full document screenshot of the current window as a binary data.