Fix default value of XSECURELOCK_SHOW_DATETIME in README.md
Fix default value of XSECURELOCK_SHOW_DATETIME in README.md

README said it was enabled, but it's disabled by default.

Mainly because some people found it annoying to include the seconds there, but there's no portable way to remove them.

Fixes #57.
google/xsecurelock,google/xsecurelock