fixing error message to conform to others

should be doc.loadInfo not sheet.loadInfo

lib/GoogleSpreadsheet.js

@@ -172,7 +172,7 @@ class GoogleSpreadsheet {
   }
 
   _ensureInfoLoaded() {
-    if (!this._rawProperties) throw new Error('You must call `sheet.loadInfo()` before accessing this property');
+    if (!this._rawProperties) throw new Error('You must call `doc.loadInfo()` before accessing this property');
   }
 
   _updateRawProperties(newProperties) { this._rawProperties = newProperties; }