Removed Call binding (some-prop.call="...") in documentation

[ivanbacher]

Adding a .call binding will result is the following error:

The ".call" binding command has been removed in v2. If you want to pass a callback that preserves the context of the function call, you can use lambda instead. Refer to lambda expression doc for more details.

Hence, I think it makes sense to remove it from the migrating to Aurelia 2 section

[bigopon]

Thanks @ivanbacher