variable misnamed as "filepath" instead of "file_path" in README.rst

[Atishaysjain]

In the following code in the readme file:

from snapgene_reader import snapgene_file_to_dict, snapgene_file_to_seqrecord

file_path = './snap_gene_file.dna'
dictionnary = snapgene_file_to_dict(filepath)
seqrecord = snapgene_file_to_seqrecord(filepath)

the following two lines can be changed from:

dictionnary = snapgene_file_to_dict(filepath)
seqrecord = snapgene_file_to_seqrecord(filepath)

to

dictionnary = snapgene_file_to_dict(file_path)
seqrecord = snapgene_file_to_seqrecord(file_path)

I can create a pull request and make the changes. Thank You.