Add additional comment to explain 03_if/if3 solution

[sirbuig]

I think 03_if/if3.rs is a bit ambiguous. The code to be modified looks like this:

// TODO: Fix the compiler error in the statement below.
    let identifier = if animal == "crab" {
        1
    } else if animal == "gopher" {
        2.0
    } else if animal == "snake" {
        3
    } else {
        "Unknown"
    };

which is intuitively indicating to change "Unknown" to an unused identifier, however it still won't work because the compiler will continue prompting for changes until 2.0 is replaced with 2.

Personally it took me a while to figure it out, and that's not the problem, but when I checked the official solution:

let identifier = if animal == "crab" {
        1
    } else if animal == "gopher" {
        2
    } else if animal == "snake" {
        3
    } else {
        // Any unused identifier.
        4
    };

the only change mentioned is from Unknown to 4, but there's no explanation that 2.0 must also be changed to 2. I think adding a comment to clarify the necessary type consistency would be very helpful.