Remove confusing sentence to clarify reference capabilities section
Remove confusing sentence to clarify reference capabilities section

The following sentence at the end of the document was confusing: "So
the default for any mutable reference capability is iso and the
default for any immutable reference capability is val." After some
discussion we decided it didn't make sense and didn't belong in this
part of the documentation, so I've removed it.

rurban/pony-tutorial,jkleiser/pony-tutorial