Fix default value to work with Kernel#exit

Author: drbrain

lib/rdoc/stats.rb

@@ -31,7 +31,7 @@ def initialize num_files, verbosity = 1
 
     @coverage_level = 0
     @doc_items = nil
-    @fully_documented = nil
+    @fully_documented = false
     @num_params = 0
     @percent_doc = nil
     @start = Time.now