fix API token file path in Readme

[cserteGT3]

When running

using GoogleSheets
client = sheets_client(AUTH_SCOPE_READONLY)

I got an exception, that

ERROR: Credentials file not found: C:\Users\username\.julia/config/google_sheets/credentials.json

I've updated the Readme with the correct path, though I'm not sure, what the "good" path is (~/.julia/google_sheets or ~/.julia/config/google_sheets

[blegat]

Same issue here, this PR should be merged