[instagram] fix regex for '/saved' URLs (fixes #1251)

The URL pattern erroneously had two '([^/?#]+)' capture groups,
which would split any username into 'usernam' for the first group
and 'e' for the ignored second group.

gallery_dl/extractor/instagram.py

@@ -424,7 +424,7 @@ class InstagramChannelExtractor(InstagramExtractor):
 class InstagramSavedExtractor(InstagramExtractor):
     """Extractor for ProfilePage saved media"""
     subcategory = "saved"
-    pattern = USER_PATTERN + r"([^/?#]+)/saved"
+    pattern = USER_PATTERN + r"/saved"
     test = ("https://www.instagram.com/instagram/saved/",)
 
     def posts(self):