Fix code snippet to discard a pointer in "naughty pointer"

[haleemur]

Description

Fix guide for versions 0.14 & 0.13

Fix typo in code snippet. Discarding the variable should be _ = &y; instead of _ = y;.

Replace var x: u16 = 0; with var x: u16 = 5; x -= 5;. Else the compiler complains with error: local variable is never mutated. The updated example also highlights that the compiler is able to detect the value of x is not 0 after a mutation.

Note that const x: u16 = 0 would have resulted in a different error message instead of panic: cast causes pointer to be null.

resolves: #275

[mlugg]

Please don't use this discard strategy; that it works is unfortunate and might change in the future. Instead, do something more obviously silly like y = y;. This is because while unused local constants sometimes have legitimate use cases, so _ = foo discards for those are okay, hiding this "local variable is never mutated" error is never correct in real Zig code, so teaching what looks like a supported way to hide it in counterproductive.

[haleemur]

Thank you for the suggestion @mlugg. I replaced _ = &y; with y = y as you suggest.

[haleemur]

Hi @Sobeston, Would you be able to take a look at this please. I have addressed @mlugg's comment above.