Updates small error in documentation

Need to use double quotes in this case for string interpolation.

README.md

@@ -1101,7 +1101,7 @@ queried table.
 ```ruby
 shirt_brands = ShirtBrand.search_by_name("Penguin")
   .joins(:shirt_sizes)
-  .group('shirt_brands.id, #{PgSearch::Configuration.alias('shirt_brands')}.rank')
+  .group("shirt_brands.id, #{PgSearch::Configuration.alias('shirt_brands')}.rank")
 ```
 
 ## ATTRIBUTIONS