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>

runtime/doc/index.txt

@@ -1172,6 +1172,8 @@ tag		command		action ~
 |:>|		:>		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