corrected the spelling of imp_url

Author: dletorey

css/types/attr.json

@@ -726,7 +726,7 @@
                 "opera_android": "mirror",
                 "safari": {
                   "version_added": false,
-                  "impl_url": "https://webkit.org/b/26609"
+                  "imp_url": "https://webkit.org/b/26609"
                 },
                 "safari_ios": "mirror",
                 "samsunginternet_android": "mirror",
@@ -770,7 +770,7 @@
                 "opera_android": "mirror",
                 "safari": {
                   "version_added": false,
-                  "impl_url": "https://webkit.org/b/26609"
+                  "imp_url": "https://webkit.org/b/26609"
                 },
                 "safari_ios": "mirror",
                 "samsunginternet_android": "mirror",