Format the help text to 80 column width (#1999)

diff --git a/lib/src/dartdoc_options.dart b/lib/src/dartdoc_options.dart
index af61792..23ed768 100644
--- a/lib/src/dartdoc_options.dart
+++ b/lib/src/dartdoc_options.dart
@@ -534,7 +534,7 @@
   Map<String, _YamlFileData> get _yamlAtCanonicalPathCache =>
       root.__yamlAtCanonicalPathCache;
 
-  final ArgParser __argParser = new ArgParser();
+  final ArgParser __argParser = new ArgParser(usageLineLength: 80);
 
   ArgParser get argParser => root.__argParser;