Clean up a comment in plugins/romega/ReflOmegaCore

Based on suggestion by @gasche.
instanciation is French, instantiation is English
"allows to", like "allowing to", is improper

It's possible that I should have removed more "allows", as many
instances of "foo allows to bar" could have been replaced by "foo bars"
(e.g., "[Qed] allows to check and save a complete proof term" could be
"[Qed] checks and saves a complete proof term"), but not always (e.g.,
"the optional argument allows to ignore universe polymorphism" should
not be "the optional argument ignores universe polymorphism" but "the
optional argument allows the caller to instruct Coq to ignore universe
polymorphism" or something similar).