[fluidmatrixinteractions] Rename wettingPhaseIdx() -> wettingPhase()
What this MR does / why does DuMux need it: With !1918 (merged) (d23929df) wettingPhaseIdx() was renamed to wettingPhase(). This fixes a remaining wettingPhaseIdx() in the fluidmatrixinteractions.
Which issue this MR fixes : fixes #847 (closed)