Change Perl_6 to Perl 6

Sometimes it's not too hard to confuse non-breaking space with an underscore
in emacs, as both are displayed as an underscore (one being different color).

doc/Language/experimental.pod6

@@ -4,9 +4,9 @@
 
 =SUBTITLE New features for brave users
 
-During Perl_6 development, new features are often made available on
+During Perl 6 development, new features are often made available on
 the main branch.  Eventually they may be incorpoated into the next
-version of Perl_6.  To use such features, one uses the C<experimental>
+version of Perl 6.  To use such features, one uses the C<experimental>
 pragma in program source code like this:
 
 =begin code