gh-40111: fix reference in btquotient.py

I'm looking at https://doi.org/10.1112/S1461157013000235, and there is
no Theorem 9. I think it should be 3.8 instead.

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preview.

URL: #40111
Reported by: Eloi Torrents
Reviewer(s): Frédéric Chapoton

Author: Release Manager

build/pkgs/configure/checksums.ini

@@ -1,3 +1,3 @@
 tarball=configure-VERSION.tar.gz
-sha1=343cae814ac674ace81de9f0af5eca6347fee347
-sha256=622cbdc34c528eb4594c553a2b8e9957d8d93fc3e439e91243985b24095217ec
+sha1=a93dfba9385785fee5bb0680b78d952eaa216ad1
+sha256=3fe296479bc7a527037131dec82145fe13ff75ab36dbdd08ff0b7208abf4f84a

build/pkgs/configure/package-version.txt

@@ -1 +1 @@
-19e94c2106f252c28c17b55f6a8db1a19b45e00b
+87f5584e0fb4be08f671ed912e7b8171020b6cd2

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::