bpo-35054: Add yet more index entries for symbols. (GH-10121) (GH-10171)

(cherry picked from commit 913876d824d969f8c7431e8a9d4610a9a11a786e)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
diff --git a/Doc/library/datetime.rst b/Doc/library/datetime.rst
index e7a0fdb..a1a60b0 100644
--- a/Doc/library/datetime.rst
+++ b/Doc/library/datetime.rst
@@ -2000,7 +2000,7 @@
 
 
 .. index::
-   single: %; datetime format
+   single: % (percent); datetime format
 
 .. _strftime-strptime-behavior: