Misleading wiki about message overrides

Not sure if introducing `mustEqual` was intentional in [1] but as there is built-in `equal` rule I think it should be replaced with `equal` or described in the other section (for peoples who scans not reads text) about overriding _native_ rules like:

``` js
var vm = {
acceptedTerms = ko.observable(false).extend({
required: true,
equal: {
params: true,
message: 'You must accept Terms and Conditions.'
},
});
}
```

[1] https://github.com/Knockout-Contrib/Knockout-Validation/wiki/Override-Error-Messages

Open the Knockout-Validation wiki page “Override Error Messages” linked in the issue and compare its use of mustEqual with the built-in equal rule. Update the misleading section or example so native-rule message overrides are described consistently, including the provided equal example.

Written by the indexing model from the issue text.