Add future_stmt to simple_stmt production list. (GH-8239)

diff --git a/Doc/reference/simple_stmts.rst b/Doc/reference/simple_stmts.rst
index 5198c04..d509700 100644
--- a/Doc/reference/simple_stmts.rst
+++ b/Doc/reference/simple_stmts.rst
@@ -25,6 +25,7 @@
               : | `break_stmt`
               : | `continue_stmt`
               : | `import_stmt`
+              : | `future_stmt`
               : | `global_stmt`
               : | `nonlocal_stmt`