Fix typo on readme (#506)

Author: FernandoAndrade83

README.md

@@ -147,7 +147,7 @@ remainingEmailsMessage(int howMany, String userName) =>
         other: 'There are $howMany emails left for $userName.')}''',
   name: 'remainingEmailsMessage',
   args: [howMany, userName],
-  desc: How many emails remain after archiving.',
+  desc: 'How many emails remain after archiving.',
   examples: const {'howMany': 42, 'userName': 'Fred'});
 
 print(remainingEmailsMessage(1, 'Fred'));