runtime(vim): Update syntax file (#13906)

Highlight :2match and :3match and add these to :help ex-cmd-index.

Signed-off-by: Doug Kearns <dougkearns@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/doc/index.txt b/runtime/doc/index.txt
index 1b012b7..d26592a 100644
--- a/runtime/doc/index.txt
+++ b/runtime/doc/index.txt
@@ -1172,6 +1172,8 @@
 |:>|		:>		shift lines one 'shiftwidth' right
 |:@|		:@		execute contents of a register
 |:@@|		:@@		repeat the previous ":@"
+|:2match|	:2mat[ch]	define a second match to highlight
+|:3match|	:3mat[ch]	define a third match to highlight
 |:Next|		:N[ext]		go to previous file in the argument list
 |:Print|	:P[rint]	print lines
 |:X|		:X		ask for encryption key