docs: readme fix, use jwt_svids instead of jwt_svid_file_name

[nstott]

the jwt config vars have changed,
the documentation in the stanza above is correct but the examples still reference the older config options
this

[nstott]

this commit seems to be where the fields changes, 8d4d3ab

[faisal-memon]

Thanks @nstott for catching this.

[nstott]

happy to help.
we're making great use of spiffe-helper w/ our federated oidc. the jwt features were really needed by us