Lower-case n is missing from charset implementation

[jamesyoungman]

The character codes in the User Handbook are not all readable. But there is a second source for this info - the drawing of the Lincoln Writer keyboard in the quarterly report document.

The key between '>' and 'p' looks like 'n'. But because of the illegibility of the Users Handbook, this code (20 octal) appears as the character '≈' (almost equal to).

This should be a simple fix, but I'm recording it here because I'm in the middle of another large change.

[jamesyoungman]

In other words this is a bug in base/src/charset.rs.