Add missing f-specifier for f-string

[berland]

The missing f caused an error in the printed comments, as in

-- Output file printed by res2df.pvt 1.3.0
--  at 2025-02-10 08:19:05.513992

PVDO
--         PRESSURE            VOLUMEFACTOR            VISCOSITY      
-- PVTNUM: 1
            10.0000000            1.0000000            1.0000000
           150.0000000            0.9000000            1.0000000
/

DENSITY
--        OILDENSITY           WATERDENSITY           GASDENSITY      
           800.0000000         1000.0000000            1.2000000 /

ROCK
--   {'PRESSURE':^21} {'COMPRESSIBILITY':^21}
           100.0000000            0.0001000 /

PVTW
--         PRESSURE            VOLUMEFACTOR         COMPRESSIBILITY          VISCOSITY           VISCOSIBILITY    
             1.0000000            1.0000000            0.0001000            0.2000000            0.0000100/