doc: user's guide pragma fixes

Two small fixes to the user's guide section on pragmas.

• Make the spelling of INLIN{,E}ABLE consistent
• Fix the markup for pragmas (previously displayed as LANGUAGE ⟨ext⟩, ⟨ext⟩, ... #-}), rather than {-# LANGUAGE ⟨ext⟩, ⟨ext⟩, ... #-})