Fix template parameter

[Dom Disc]

Hello.
I found in the documentation functions declared like this:

```D
pure @nogc @safe BigInt opAssign(T : BigInt)(T x);
```

What is the difference to declaring it like:

```D
pure @nogc @safe BigInt opAssign(BigInt x);
```

To me the first declaration seems to be unnecessarily bloated, so 
I ask myself: does it provide any kind of advantage? I can't see 
it.