_content/security/fuzz: tweak example for running file2fuzz

When running file2fuzz without any arguments it will read from stdin.
This might confuse users who copy the commands from the example,
expecting something to happen. Suggest running it with -h instead.

Change-Id: I50cc776af66af5126fb283dee228df644c6e340d
Reviewed-on: https://go-review.googlesource.com/c/website/+/401954
Reviewed-by: Daniel Martí <[email protected]>
Reviewed-by: Julie Qiu <[email protected]>
Auto-Submit: Dominik Honnef <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Run-TryBot: Dominik Honnef <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>

Author: dominikh

_content/security/fuzz/index.md

@@ -245,7 +245,7 @@ To use this tool:
 
 ```
 $ go install golang.org/x/tools/cmd/file2fuzz@latest
-$ file2fuzz
+$ file2fuzz -h
 ```
 
 ## Resources