doc: update UrlCleaner description in sentinel-web-servlet README.md (#1051)

sentinel-adapter/sentinel-web-servlet/README.md

@@ -55,8 +55,8 @@ For REST APIs, you have to clean the URL resource (e.g. `/foo/1` and `/foo/2` ->
 the amount of context and resources will exceed the threshold.
 
 If you need to exclude some URLs (that should not be recorded as Sentinel resources), you could also
-leverage the `UrlCleaner` interface. You may unify the unwanted URLs to the empty string `""`,
+leverage the `UrlCleaner` interface. You may unify the unwanted URLs to the empty string `""` or `null`,
 then the URLs will be excluded (since Sentinel 1.6.3).
 
 `RequestOriginParser` interface is useful for extracting request origin (e.g. IP or appName from HTTP Header)
-from HTTP request. You can implement your own `RequestOriginParser` and register to `WebCallbackManager`.
+from HTTP request. You can implement your own `RequestOriginParser` and register to `WebCallbackManager`.