Restore single_config_entry in manifest.json

Restored 'single_config_entry' property in manifest.

Author: hitchin999

custom_components/yidcal/manifest.json

@@ -4,7 +4,6 @@
 
   "codeowners": ["@hitchin999"],
   "config_flow": true,
-  "single_config_entry": true,
   "documentation": "https://github.com/hitchin999/YidCal",
   "iot_class": "local_polling",
   "issue_tracker": "https://github.com/hitchin999/YidCal/issues",
@@ -15,5 +14,6 @@
     "zmanim==0.3.1",
     "geopy==2.4.1"
   ],
+  "single_config_entry": true,
   "version": "0.5.7"
 }