M44945: Adding misssing word

Translator has reported possible source content issue. 
Description: There seems to be something wrong with this sentence. Assume it was supposed to say: "...with NULL values for columns THAT aren't matched." and translated accordingly.

Author: v-maudel

articles/data-factory/data-flow-join.md

@@ -31,7 +31,7 @@ Right outer join returns all rows from the right stream and matched records from
 
 ### Full Outer
 
-Full outer join outputs all columns and rows from both sides with NULL values for columns aren't matched.
+Full outer join outputs all columns and rows from both sides with NULL values for columns that aren't matched.
 
 ### Cross Join