Wrong log levels in the manual

[lbonn]

The documentation for the log levels is incoherent with the definitiosn in libcore/core.rc: they are numbered from 0 to 3 in the docs and from 1 to 4 in the code.

http://static.rust-lang.org/doc/0.5/rust.html#logging-system
https://github.com/mozilla/rust/blob/master/src/libcore/core.rc#L207

[catamorphism]

Fixed in e856606