README: Use recommended RFC 2119 wording

Match the recommendation in the RFC's abstract [1].  Also bump
"keywords" -> "key words" in the subsequent C99 sentence for
consistency.

[1]: https://tools.ietf.org/html/rfc2119

Signed-off-by: W. Trevor King <wking@tremily.us>

Author: wking

README.md

@@ -23,9 +23,9 @@ Table of Contents
     - [Windows-specific Configuration](config-windows.md)
 - [Glossary](glossary.md)
 
-In the specifications in the above table of contents, the keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as described in [RFC 2119](http://tools.ietf.org/html/rfc2119) (Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997).
+The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as described in [RFC 2119][rfc2119].
 
-The keywords "unspecified", "undefined", and "implementation-defined" are to be interpreted as described in the [rationale for the C99 standard][c99-unspecified].
+The key words "unspecified", "undefined", and "implementation-defined" are to be interpreted as described in the [rationale for the C99 standard][c99-unspecified].
 
 An implementation is not compliant for a given CPU architecture if it fails to satisfy one or more of the MUST, REQUIRED, or SHALL requirements for the protocols it implements.
 An implementation is compliant for a given CPU architecture if it satisfies all the MUST, REQUIRED, and SHALL requirements for the protocols it implements.
@@ -168,3 +168,4 @@ Read more on [How to Write a Git Commit Message](http://chris.beams.io/posts/git
 [c99-unspecified]: http://www.open-std.org/jtc1/sc22/wg14/www/C99RationaleV5.10.pdf#page=18
 [UberConference]: https://www.uberconference.com/opencontainers
 [irc-logs]: http://ircbot.wl.linuxfoundation.org/eavesdrop/%23opencontainers/
+[rfc2119]: http://tools.ietf.org/html/rfc2119