Fix documentation

[stweil]

I noticed the typo while preparing new training tests (see https://github.com/UB-Mannheim/AustrianNewspapers/wiki/Training-with-Calamari) with disappointing performance on Apple M1 with GPU.

[andbue]

Thanks! Interesting to see your benchmarks. I would have expected at least some improvements there, but the "ETA" value after 100 steps looks as if it's almost slower on GPU than on CPU...

[stweil]

It's much slower on M1 GPU than on CPU. M1 CPU is a good choice compared to Intel CPU, but M1 GPU is currently a bad choice. I wonder why, because I had expected that Apple's unified memory which avoids data transfer between CPU and GPU RAM would result in a much better performance.