Fix pub outdated help messages consistency (#2408)

diff --git a/lib/src/command/outdated.dart b/lib/src/command/outdated.dart
index 5826688..6ff24bb 100644
--- a/lib/src/command/outdated.dart
+++ b/lib/src/command/outdated.dart
@@ -40,11 +40,11 @@
 
     argParser.addFlag('up-to-date',
         defaultsTo: false,
-        help: 'Include dependencies that are already at the latest version');
+        help: 'Include dependencies that are already at the latest version.');
 
     argParser.addFlag('pre-releases',
         defaultsTo: false,
-        help: 'Include pre-releases when reporting latest version');
+        help: 'Include pre-releases when reporting latest version.');
 
     argParser.addFlag(
       'dev-dependencies',