gocr: 0.50 -> 0.51

Semi-automatic update. These checks were done:

- built on NixOS
- ran `/nix/store/6mm4a7nrr7ahv7wc47c137f28frhkrm5-gocr-0.51/bin/gocr --help` got 0 exit code
- ran `/nix/store/6mm4a7nrr7ahv7wc47c137f28frhkrm5-gocr-0.51/bin/gocr --help` and found version 0.51
- found 0.51 with grep in /nix/store/6mm4a7nrr7ahv7wc47c137f28frhkrm5-gocr-0.51
- found 0.51 in filename of file in /nix/store/6mm4a7nrr7ahv7wc47c137f28frhkrm5-gocr-0.51