Budget Minimum Proportions must be > 0

The documentation says that the minimum spend proportion for each channel should be between 0 and 1. Whilst technically this is correct, it would be useful to make it clear that (currently) the minimum value must be > 0, else you get the following error:

```
Error in step_2_ads_spend(my_variables = my_variables, campaign_spend_mean = weekly_spend, :
You entered a negative proportion of spend on a channel. Enter a proportion between 0 and 1
```

An alternative option (and better option, in my opinion) would be to allow zero spend to be a valid value.