docs: Fix link to toolchain configuration doc in side bar (#1866)

The index had the wrong doc name, so it wasn't showing up in the
rendered docs.

docs/sphinx/index.md

diff --git a/docs/sphinx/index.md b/docs/sphinx/index.md
index d7dbb44..eadd3ac 100644
--- a/docs/sphinx/index.md
+++ b/docs/sphinx/index.md
@@ -57,7 +57,7 @@
 self
 getting-started
 pypi-dependencies
-configuring
+toolchains
 pip
 coverage
 gazelle