fix: wording

Author: gadomski

README.md

@@ -98,7 +98,7 @@ example:
     if `properties` is included and `properties.datetime` is excluded, then
     `datetime` should not be in `properties`, but every other nested field should be.
 6. If the same field is present in both `include` and `exclude`, it should be included.
-7. If the a field is not present in `include`, but it is in `exclude`, it should be excluded.
+7. If a field is not present in `include`, but it is present in `exclude`, it should be excluded.
 
 ### `null` vs. empty vs. missing