very minor nit - update "an new" to "a new" (#519)

README.md

@@ -772,7 +772,7 @@ from {{ ref('my_model') }}
 This macro unions together an array of [Relations](https://docs.getdbt.com/docs/writing-code-in-dbt/class-reference/#relation),
 even when columns have differing orders in each Relation, and/or some columns are
 missing from some relations. Any columns exclusive to a subset of these
-relations will be filled with `null` where not present. An new column
+relations will be filled with `null` where not present. A new column
 (`_dbt_source_relation`) is also added to indicate the source for each record.
 
 **Usage:**