docs: fix link in openapi description (#345)

src/gene/main.py

@@ -16,7 +16,7 @@
 The Gene Normalizer provides tools for resolving ambiguous gene references to
 consistently-structured, normalized terms.
 
-See the [documentation](https://gene-normalizer.readthedocs.io/en/latest/) for more
+See the [documentation](https://gene-normalizer.readthedocs.io/latest/) for more
 information.
 """