Fix docs for -dim

The docs said Discard...by Seq, but the examples suggest this is by
Method instead.  The name of the option in the code is DiscardMethod,
so I updated the docs.

Signed-off-by: Richard Laager <rlaager@wiktel.com>

README.md

@@ -50,7 +50,7 @@ docker build --no-cache -t sipcapture/heplify:latest -f docker/heplify/Dockerfil
   -di string
 	Discard uninteresting packets by any string
   -dim string
-	Discard uninteresting SIP packets by CSeq [OPTIONS,NOTIFY]
+	Discard uninteresting SIP packets by Method [OPTIONS,NOTIFY]
   -diip string
 	Discard uninteresting SIP packets by Source or Destination IP(s)
   -disip string

main.go

@@ -52,7 +52,7 @@ func createFlags() {
 	flag.BoolVar(&config.Cfg.SkipVerify, "skipverify", false, "skip certifcate validation")
 	flag.BoolVar(&config.Cfg.Dedup, "dd", false, "Deduplicate packets")
 	flag.StringVar(&config.Cfg.Discard, "di", "", "Discard uninteresting packets by any string")
-	flag.StringVar(&config.Cfg.DiscardMethod, "dim", "", "Discard uninteresting SIP packets by CSeq [OPTIONS,NOTIFY]")
+	flag.StringVar(&config.Cfg.DiscardMethod, "dim", "", "Discard uninteresting SIP packets by Method [OPTIONS,NOTIFY]")
 	flag.StringVar(&config.Cfg.DiscardIP, "diip", "", "Discard uninteresting SIP packets by Source or Destination IP(s)")
 	flag.StringVar(&config.Cfg.DiscardSrcIP, "disip", "", "Discard uninteresting SIP packets by Source IP(s)")
 	flag.StringVar(&config.Cfg.DiscardDstIP, "didip", "", "Discard uninteresting SIP packets by Destination IP(s)")