doc: fix log level ids

doc/rust.md

@@ -2472,7 +2472,7 @@ disabled dynamically at run-time on a per-task and per-item basis. See
 Each `log` expression must be provided with a *level* argument in
 addition to the value to log. The logging level is a `u32` value, where
 lower levels indicate more-urgent levels of logging. By default, the lowest
-four logging levels (`0_u32 ... 3_u32`) are predefined as the constants
+four logging levels (`1_u32 ... 4_u32`) are predefined as the constants
 `error`, `warn`, `info` and `debug` in the `core` library.
 
 Additionally, the macros `error!`, `warn!`, `info!` and `debug!` are defined
@@ -3242,10 +3242,10 @@ level will be output to the console.
 The path to a module consists of the crate name, any parent modules,
 then the module itself, all separated by double colons (`::`).  The
 optional log level can be appended to the module path with an equals
-sign (`=`) followed by the log level, from 0 to 3, inclusive. Level 0
-is the error level, 1 is warning, 2 info, and 3 debug. Any logs
+sign (`=`) followed by the log level, from 1 to 4, inclusive. Level 1
+is the error level, 2 is warning, 3 info, and 4 debug. Any logs
 less than or equal to the specified level will be output. If not
-specified then log level 3 is assumed.
+specified then log level 4 is assumed.
 
 As an example, to see all the logs generated by the compiler, you would set
 `RUST_LOG` to `rustc`, which is the crate name (as specified in its `link`