rm duplicate explanation (#3780)

Author: naruse666

site-src/guides/http-header-modifier.md

@@ -1,7 +1,5 @@
 # HTTP Header Modifiers
 
-[HTTPRoute resources](../api-types/httproute.md) can modify the headers of HTTP requests and the HTTP responses from clients.
-There are two types of [filters](../api-types/httproute.md#filters-optional) available to meet these requirements: `RequestHeaderModifier` and `ResponseHeaderModifier`.
 [HTTPRoute resources](../api-types/httproute.md) can modify the headers of HTTP requests and the HTTP responses from clients.
 There are two types of [filters](../api-types/httproute.md#filters-optional) available to meet these requirements: `RequestHeaderModifier` and `ResponseHeaderModifier`.