Add an index for \ (unspace)

Author: titsuki

doc/Language/syntax.pod6

@@ -56,7 +56,7 @@ if True {say "Hello"}
 
 though you can't leave out any of the remaining whitespace.
 
-=head2 Unspace
+=head2 X<Unspace|syntax,\>
 
 In many places where the compiler would not allow a space you can use any
 whitespace that is quoted with a backslash. Unspaces in tokens are not