[libc++][ranges] Sets ranges feature-test macro.

D131234 marked the ranges papers as complete, but it didn't set the
feature-test macro.

Reviewed By: ldionne, var-const, #libc

Differential Revision: https://reviews.llvm.org/D131326

NOKEYCHECK=True
GitOrigin-RevId: 195287d90afb1b97a0fb869da501370e7c3e67c6
diff --git a/docs/FeatureTestMacroTable.rst b/docs/FeatureTestMacroTable.rst
index 9aa183a..e5a06dc 100644
--- a/docs/FeatureTestMacroTable.rst
+++ b/docs/FeatureTestMacroTable.rst
@@ -262,7 +262,7 @@
     ------------------------------------------------- -----------------
     ``__cpp_lib_polymorphic_allocator``               *unimplemented*
     ------------------------------------------------- -----------------
-    ``__cpp_lib_ranges``                              *unimplemented*
+    ``__cpp_lib_ranges``                              ``201811L``
     ------------------------------------------------- -----------------
     ``__cpp_lib_remove_cvref``                        ``201711L``
     ------------------------------------------------- -----------------