webp-container-spec.txt: update 'key words' text
webp-container-spec.txt: update 'key words' text

RFC 2119 was updated by RFC 8174; use the text from there

+ change INFORMATIVE to informative to avoid confusion with the key
  words in the RFCs

Change-Id: I0a3fe9dc48d284e7ac95633896ffb855ecd1a229

webmproject/libwebp,webmproject/libwebp,webmproject/libwebp,webmproject/libwebp,webmproject/libwebp