fix typo: missing comma

Author: uvtc

doc/Language/containers.pod

@@ -229,7 +229,7 @@ a thing happening accidentally.
 The C<%> and C<@> sigils in Perl 6 generally indicate multiple values
 to an iteration construct, whereas the C<$> sigil indicates only one value.
 
-    my @a = 1, 2 3;
+    my @a = 1, 2, 3;
     for @a { };         # 3 iterations
     my $a = (1, 2, 3);
     for $a { };         # 1 iteration