fix read_x_ar_theory_vec to use proper Bbl

[zatkins2]

pspipe_utils/pspipe_utils/covariance.py

Line 745 in bd1232c

Bbl = np.load(f"{mcm_dir}/{spec_name}_Bbl_spin0xspin0.npy")

Using proper Bbl is easy and one less thing to worry about. Adding this issue just to not forget to do this.