fix typo in the link (#110)

README.rst

@@ -1,4 +1,4 @@
 Scanpy tutorials
 ================
 
-See this `page <https://scanpy.readthedocs.io/en/latest/_tutorials.html>`__ for more context.
+See this `page <https://scanpy.readthedocs.io/en/latest/tutorials.html>`__ for more context.