Union types

[rusty-bill]

The second sentence under Union types reads:

Since the accessing the wrong field can cause unexpected or undefined behaviour, unsafe is required to read from a union field or to write to a field that doesn't implement Copy.

Removing the first 'the' would make it read more clearly. Also, using the American English spelling of 'behavior' would be consistent with the rest of the documentation.

[ehuss]

I'm going to close since the wording was changed in #478 (and various other following PRs.) Thanks!