Update RAM figure in OS readme.

os/README.mkdn

@@ -2,7 +2,7 @@
 
 This is a wee operating system written to support the `async` style of
 programming in Rust on microcontrollers. It fits in about 2 kiB of Flash and
-uses about 20 bytes of RAM (before your tasks are added). In that space, you get
+uses about 40 bytes of RAM (before your tasks are added). In that space, you get
 a full `async` runtime with multiple tasks, support for complex concurrency via
 `join` and `select`, and a lot of convenient but simple APIs. (If you want to
 see what a `lilos` program looks like, look in the `examples` directory, or read