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

(cherry picked from commit cdb96f45b61a40a7e7c4c83b4b1f14ef6f5cf4fa)

Co-authored-by: Andrés Delfino <adelfino@gmail.com>
diff --git a/Doc/reference/simple_stmts.rst b/Doc/reference/simple_stmts.rst
index bc9daae..be2673b 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`