remove @ so that "since" script will work

HOWTO/DOCUMENTATION.md

@@ -447,7 +447,7 @@ Get information about a thing.
 As the actual version is generally not yet certain at the time when a
 change gets merged, the an internal ticket number (for example
 `OTP-12345`) to use as a placeholder in the respective `since` tags,
-such as `-doc #{since => ~"OTP @OTP-12345@"}.`. When your new
+such as `-doc #{since => ~"OTP OTP-12345"}.`. When your new
 functions are released with an Erlang/OTP release, this placeholder
 will get replaced with the actual OTP version, leading to something
 like "OTP 26.0".