fix reference in btquotient.py

Author: Eloitor

src/sage/modular/btquotients/btquotient.py

@@ -1714,7 +1714,7 @@ def _compute_invariants(self):
         Compute certain invariants from the level data of the quotient
         which allow one to compute the genus of the curve.
 
-        Details to be found in Theorem 9 of [FM2014]_.
+        Details to be found in Theorem 3.8 of [FM2014]_.
 
         EXAMPLES::