doc: fix references to error keyword

References for err.signal and err.code should be error.signal and
error.code.

Fixes joyent/node#6862

doc/api/child_process.markdown

@@ -510,8 +510,8 @@ Runs a command in a shell and buffers the output.
     });
 
 The callback gets the arguments `(error, stdout, stderr)`. On success, `error`
-will be `null`.  On error, `error` will be an instance of `Error` and `err.code`
+will be `null`.  On error, `error` will be an instance of `Error` and `error.code`
-will be the exit code of the child process, and `err.signal` will be set to the
+will be the exit code of the child process, and `error.signal` will be set to the
 signal that terminated the process.
 
 There is a second optional argument to specify several options. The