You're misreading it, it says "IV chord (C7)."

12 bar blues is based around a I-IV-V progression. In this case, the key is G.

I is G

IV up from G is C

V from G is D

So now you have your chord names, G7 G-B-D-F

C7: C-E-G-Bb

D7: D-F#-A-C