.readthedocs.yml: fix type docs/

Out documentation directory is doc/ and not docs/.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/.readthedocs.yml b/.readthedocs.yml
index f3fb5ed..44949ea 100644
--- a/.readthedocs.yml
+++ b/.readthedocs.yml
@@ -7,7 +7,7 @@
 
 # Build documentation in the docs/ directory with Sphinx
 sphinx:
-  configuration: docs/conf.py
+  configuration: doc/conf.py
 
 # Optionally build your docs in additional formats such as PDF and ePub
 formats: []