chore: fix typo in FrobeniusRiou.lean (#180)

Co-authored-by: euprunin <[email protected]>

Author: euprunin

FLT/MathlibExperiments/FrobeniusRiou.lean

@@ -675,7 +675,7 @@ variable [Nontrivial B]
 
 
 -- not sure if we need this but let's prove it just to check our setup is OK.
--- The claim is that the preimage of Q really is P (rather than just contining P)
+-- The claim is that the preimage of Q really is P (rather than just containing P)
 -- and the proof is that A/P -> B/Q extends to a map of fields which is injective,
 -- so A/P -> B/Q must also be injective.
 open IsScalarTower in