fix documentation

lib/msf/core/exploit.rb

@@ -1276,7 +1276,7 @@ class Exploit < Msf::Module
   ##
 
   #
-  # The reason why the exploit was not successful (one of Msf::Exploit::FailReason)
+  # The reason why the exploit was not successful (one of Msf::Module::Failure)
   #
   attr_accessor  :fail_reason
 
@@ -1393,4 +1393,3 @@ protected
 end
 
 end
-