docs(wpcarro/blog): Use "today" in the call to date

Now I can copy-paste without replacing the placeholder
date-string (i.e. "2020-04-02").

Change-Id: Ie4ee843b495447a2d4b7e6634114b81b3fc0a50e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/4822
Tested-by: BuildkiteCI
Reviewed-by: wpcarro <wpcarro@gmail.com>

users/wpcarro/website/blog/posts.nix

@@ -1,5 +1,5 @@
 # To format the date, run:
-#   $ date -d "2020-04-02" +%s
+#   $ date -d "today" +%s
 [
   {
     key = "cell-phone-experiment";