patch 8.2.1635: no digraph for 0x2022 BULLET

Problem:    No digraph for 0x2022 BULLET.
Solution:   Use "oo". (Hans Ginzel, closes #6904)
diff --git a/runtime/doc/digraph.txt b/runtime/doc/digraph.txt
index e24f7e9..79c7cf0 100644
--- a/runtime/doc/digraph.txt
+++ b/runtime/doc/digraph.txt
@@ -921,6 +921,7 @@
 ‟	9"	201F	8223	DOUBLE HIGH-REVERSED-9 QUOTATION MARK
 †	/-	2020	8224	DAGGER
 ‡	/=	2021	8225	DOUBLE DAGGER
+•	oo	2022	8226	BULLET
 ‥	..	2025	8229	TWO DOT LEADER
 …	,.	2026	8230	HORIZONTAL ELLIPSIS
 ‰	%0	2030	8240	PER MILLE SIGN