Make the "undef" message refer to Any, rather than Mu

As suggested at http://irclog.perlgeek.de/perl6/2013-06-14#i_7197427

src/Perl6/Grammar.nqp

@@ -1405,7 +1405,7 @@ grammar Perl6::Grammar is HLL::Grammar does STD {
         [ <?before [ '(' || \h*<sigil><twigil>**0..1\w ] >
             <.obs('undef as a verb', 'undefine function or assignment of Nil')>
         ]?
-        <.obs('undef as a value', "something more specific:\n\tMu (the \"most undefined\" type object),\n\tan undefined type object such as Int,\n\t:!defined as a matcher,\n\tAny:U as a type constraint,\n\tNil as the absense of a value\n\tor fail() as a failure return\n\t   ")>
+        <.obs('undef as a value', "something more specific:\n\tAny (the \"whatever\" type object),\n\tan undefined type object such as Int,\n\t:!defined as a matcher,\n\tAny:U as a type constraint,\n\tNil as the absense of a value\n\tor fail() as a failure return\n\t   ")>
     }
 
     token term:sym<new> {