Nav: change Reading Guide icon to glasses

[bryceerobertson]

[bryceerobertson]

I think it represents it better. I'm assuming this is relatively quick to do?

[melissasamworth]

@ricewithabee I would prefer not doing this, as I want to be the only one to do these illustrations (for consistency) and they probably take me 30 minutes total which is... time I could be doing more useful stuff. My preference is to take feedback like this and put it in backburner for a time when it might be highest priority. Open to other ideas though.