Fix incorrect syntax in broadcasting example.

[dlyongemallo]

Summary

The example given for broadcasting uses the incorrect syntax for declaring quantum registers. This is a minor edit to fix the typo.

Details and comments

The correct syntax (see, e.g., here, or even just down below in this very same doc) is:

qubit[SIZE] q;

The current documentation incorrectly puts the size after the register name:

qubit q[SIZE];

[jakelishman]

Thanks!