Fix typo on readme (#506)

diff --git a/README.md b/README.md
index 0b80e67..1b5836b 100644
--- a/README.md
+++ b/README.md
@@ -147,7 +147,7 @@
         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'));