Update plugins.asciidoc

Added Vietnamese Analyser to plugins page

Closes elastic#6647

Author: clintongormley

docs/reference/modules/plugins.asciidoc

@@ -199,6 +199,7 @@ You can disable that check using `plugins.check_lucene: false`.
 * https://github.com/synhershko/elasticsearch-analysis-hebrew[Hebrew Analysis Plugin] (by Itamar Syn-Hershko)
 * https://github.com/medcl/elasticsearch-analysis-pinyin[Pinyin Analysis Plugin] (by Medcl)
 * https://github.com/medcl/elasticsearch-analysis-string2int[String2Integer Analysis Plugin] (by Medcl)
+* https://github.com/duydo/elasticsearch-analysis-vietnamese[Vietnamese Analysis Plugin] (by Duy Do)
 
 [float]
 [[discovery-plugins]]