Update stopwords.txt

In brazillian portuguese the conjuntion "em(preposition)+<a|o|as|os>(article)" take the form "na, nas, no, nos" being commom stop words.

For some reason the "nas" conjunction appear twice and the "no" is nowhere to be found, I think it was probably a mistake.

This pull request add the word to the list and remove the duplication.

lucene/analysis/common/src/resources/org/apache/lucene/analysis/br/stopwords.txt

@@ -68,7 +68,7 @@ mesmas
 mesmo
 mesmos
 na
-nas
+no
 nao
 nas
 nem