Fix code examples in the README

[VPetukhov]

The current examples don't work. It should be:

from genevector.data import GeneVectorDataset
from genevector.model import GeneVector
from genevector.embedding import GeneEmbedding, CellEmbedding

import scanpy as sc

dataset = GeneVectorDataset(adata, device="cuda")

cmps = GeneVector(dataset,
                  output_file="genes.vec",
                  emb_dimension=100,
                  device="cuda")
cmps.train(1000, threshold=1e-6) # run for 1000 iterations or loss delta below 1e-6.

[nceglia]

Thanks so much!