Update cookbook/01strings/01-00introduction.pl

The code:
<code>my $string = "1" ~ "1" + 10;</code>
will actually output "111" in Perl6.

Since in Perl5, "~" (actually ".") is the same precedence as "+",
but in Perl6, numeric operations have higher precedence than string operations.
So the "1" + 1 part will be evaluated first, giving "111" as the result.

Just a small modification here. :)