The example in "Internal log rotation" includes the tuple
{name, "error.log"}

This apparently should be

{file, "error.log"}

as seen in other config examples in README.md.

Fixed forgotten commas.

The current set of instructions mention 'crash_log' being left undefined
as a variable to disable it, but when that happens, lager starts it at a
default location.

This commit makes it obvious that the value must explicitly set to
'undefined' in order to disable crash_log.