Two small changes for "RIF Core Dialect"

Hi all.

In Section 1:
"RIF-Core is based on the built-ins of RIF-DTB 1.0"

For an overview section, it might be better to make clear
what such "built-ins" are.

In Section 2.2:

"In Core there" ->
"In RIF-Core, there"

Greetings
Sven Hartrumpf