Fixed error in my correction

Changed from `an username` to `a username`

modules/exploits/windows/smb/ms17_010_eternalblue.rb

@@ -678,7 +678,7 @@ class MetasploitModule < Msf::Exploit::Remote
   end
 
   # Returns the value to be passed to SMB clients for
-  # the username. If the user has not supplied an username
+  # the username. If the user has not supplied a username
   # it returns an empty string to trigger an anonymous
   # logon.
   #