Fixup out of date reference to rtt.get

[TyOverby]

The overview currently contains this section (emphasis my own):

A runtime type is an expression of type rtt <type>, which is another form of opaque value type. It represents the static type <type> at runtime. In its plain form, a runtime type is obtained using the instruction rtt.get

(rtt.canon <type>)

I believe that the text "rtt.get" was intended to be replaced by "rtt.canon"