"Parallel fifths" means "parallel perfect fifths". A move from a perfect fifth to a diminished fifth, as in the textbook example's C-G (P5) to B-F (d5), is permitted.
Another option would be to double the fifth in the first chord.
X: 1 T: Harmonizing 5-4-3 with I-V-I K: C M: none L: 1/2 [V:V1] [Geg] [Bdf] [Gce] [V:V2 clef=bass] C G, C
This seems to be keyboard style, not the SATB 4-voice harmony more usually found in harmony exercises. So we don't have to keep to the voice-leading conventions of a Bach chorale harmonisation!
Block chords in similar motion, and the unmelodic 1-5 bass line are fine in this style. In a more vocal style, I'd want that B in the second chord to resolve up to the tonic, C, or at least resolve down by step. But, I repeat, this ISN'T vocal writing. It doesn't have the rich texture of 4 independent voices. But it has its own strength. And, anyway, it's only consecutive PERFECT 5ths that are forbidden.