The way I see it, you have two choices:
Do nothing - I do not endorse this, as it transmits the message that you do not care, or even worse, you did it on purpose;
Send another e-mail with the proper apologies (do not over-do it), and correct the mistake.
The message can be as simple as:
Dear ....
In previous e-mail I wrote: "...ABC..." while I intended to write "...XYZ...". Please accept my apologies, this was totally unintended. I cannot even explain it to myself.
Yours...
Words may vary.
