feat: updated generics1

[poneciak57]

Updated generics1.rs.
Now it teaches how to define function that uses generics.

closes #1448

[mo8it]

generics1 is fixed in the upcoming version 6. But we can think about adding something like this as a third generics exercise.

[poneciak57]

ok i will add this as a third one

[poneciak57]

U know what, I will create new PR instead of merging anything after that time.