Update main.rs

[Codimow]

fixed use of deprecated function

[rickrain]

Nice catch @Codimow. However, there are several references throughout the book that will also need to be updated for this change, which requires v9.0 or higher of the rand crate to use this syntax. Later in ch. 2, which is where your PR is focused, there is text later in the chapter about semantic versioning and the cargo.lock file, suggesting that this was intentional - the "book" does say to use a specific version which supports subsequent references to it.

Anyway, I believe that this change, as-is, will actually introduce confusion unless other parts of the book are updated and re-worded to support it. For example, there are references in chapters 1, 7, 9 ,and 14 that I know of. Could be more but that's what I found doing a quick search.

[carols10cents]

This is a duplicate of #4254. Thanks though!