hw01: 2nd param of filter function

[le-n-qui]

I have been studying Rust programming lately, and come across this wonderful resource from cis198-2016f. Currently, I am using Rust 1.38. When I try to compile problem1.rs, there is a problem with the second parameter of filter function. Rust has changed a lot since 2016, and I want to bring attention to this issue. What I decided to do was to change the parameter with function pointer fn (I read about this in section 19.4 of the online Rust programming book). Maybe function pointer would be the right replacement.