Update n-api.md

Author: jiacai2050

doc/api/n-api.md

@@ -261,7 +261,7 @@ For example v18.15.0 supports only Node-API version 8. ABI stability was
 achieved because 8 was a strict superset of all previous versions.
 
 As of version 9, while Node-API versions continue to be versioned
-independently an add-on that ran with Node-API version 9 may need
+independently, an add-on that ran with Node-API version 9 may need
 code updates to run with Node-API version 10. ABI stability
 is maintained, however, because Node.js versions that support
 Node-API versions higher than 8 will support all versions