about.html: when was Android M again?

M was long enough ago that even I don't remember exactly when, and I
work on Android! The year is probably more meaningful to most people.

www/about.html

diff --git a/www/about.html b/www/about.html
index 43cd2cc..cb37e4f 100755
--- a/www/about.html
+++ b/www/about.html
@@ -16,7 +16,7 @@
 (<a href=http://landley.net/talks/celf-2013.txt>outline</a>,
 <a href=https://www.youtube.com/watch?v=SGmtP5Lg_t0>video</a>), Google
 <a href=https://lwn.net/Articles/629362/>merged toybox into AOSP</a> and
-began shipping toybox in Android Marshmallow.</p>
+began shipping toybox in Android Marshmallow in 2015.</p>
 
 <p>Toybox aims to provide one quarter of a theoretical "minimal native
 development environment", which is the simplest Linux system capable of