Fix template parameter

[bauss]

On Monday, 8 August 2022 at 12:02:02 UTC, Dom Disc wrote:
>
> ```D
> pure @nogc @safe BigInt opAssign(T : BigInt)(T x);
> ```
>

This will only be included in the object file if used.

> What is the difference to declaring it like:
>
> ```D
> pure @nogc @safe BigInt opAssign(BigInt x);
> ```
>

This will always be in the object file.

Someone can correct me if I'm wrong, but I think it's mostly 
linkage optimization.