Fix mention of date_time in the readme

Author: jeking3

README.md

@@ -34,4 +34,4 @@ Distributed under the [Boost Software License, Version 1.0](http://www.boost.org
 * [Ask questions](http://stackoverflow.com/questions/ask?tags=c%2B%2B,boost,boost-program_options): Be sure to read the documentation first to see if it answers your question.
 * [Report bugs](https://github.com/boostorg/program_options/issues): Be sure to mention Boost version, platform and compiler you're using. A small compilable code sample to reproduce the problem is always good as well.
 * [Submit Pull Requests](https://github.com/boostorg/program_options/pulls) against the **develop** branch. Note that by submitting patches you agree to license your modifications under the [Boost Software License, Version 1.0](http://www.boost.org/LICENSE_1_0.txt).  Be sure to include tests proving your changes work properly.
-* Discussions about the library are held on the [Boost developers mailing list](http://www.boost.org/community/groups.html#main). Be sure to read the [discussion policy](http://www.boost.org/community/policy.html) before posting and add the `[date_time]` tag at the beginning of the subject line.
+* Discussions about the library are held on the [Boost developers mailing list](http://www.boost.org/community/groups.html#main). Be sure to read the [discussion policy](http://www.boost.org/community/policy.html) before posting and add the `[program_options]` tag at the beginning of the subject line.