HTTP2: Fix README about change of nghttp2

nghttp2ver.h is a local header, so use "" not <>

See commit 4a182d48b1105889b51ee3bb06b033c636965130

Change-Id: I49210906290210fde8f496996e0e5ae69d697338
Reviewed-on: https://code.wireshark.org/review/3349
Reviewed-by: Evan Huus <eapache@gmail.com>