Fix SQL stored procedure syntax to use SELECT instead of RETURN

In SQL Server, RETURN is intended only for integer status codes so that SELECT is the correct for returning computed string results from a stored procedure

Author: srdrtmy

entity-framework/core/managing-schemas/migrations/managing.md

@@ -151,7 +151,7 @@ migrationBuilder.Sql(
         @LastName nvarchar(50),
         @FirstName nvarchar(50)
     AS
-        RETURN @LastName + @FirstName;')");
+        SELECT @LastName + @FirstName;')");
 ```
 
 > [!TIP]