Ignore a doctest again

I accidentally typoed `no_run`, but compiling the test fails since the
`device.x` path is wrong. I don't think this can be fixed, so ignore the
test instead.

Author: jonas-schievink

cortex-m-rt/src/lib.rs

@@ -342,7 +342,7 @@
 //! must contain build script that puts `device.x` somewhere the linker can find. An example of such
 //! build script is shown below:
 //!
-//! ```no_ruin
+//! ```ignore
 //! use std::env;
 //! use std::fs::File;
 //! use std::io::Write;