update httpx doc for -filter-duplicates option (#117)

Author: dogancanbakir

tools/httpx/usage.mdx

@@ -63,6 +63,7 @@ EXTRACTOR:
 FILTERS:
    -fc, -filter-code string            filter response with specified status code (-fc 403,401)
    -fep, -filter-error-page            filter response with ML based error page detection
+   -fd, -filter-duplicates             filter out near-duplicate responses (only first response is retained)
    -fl, -filter-length string          filter response with specified content length (-fl 23,33)
    -flc, -filter-line-count string     filter response body with specified line count (-flc 423,532)
    -fwc, -filter-word-count string     filter response body with specified word count (-fwc 423,532)