Fix syntax error in example in documentation.

[Eren121]

Example was not syntaxically correct:

int vec(3);
vec[0] = 0; vec[1] = 1; vec[2] = 2;
thrust::for_each(thrust::host, vec.begin(), vec.end(), printf_functor());

[GPUtester]

Can one of the admins verify this patch?

Admins can comment ok to test to allow this one PR to run or add to allowlist to allow all future PRs from the same author to run.

[miscco]

Thank a lot for finding and fixing the invalid documentation.