Change multi JWK Set min from 2 to 1

[MicahParks]

The purpose of this pull request is to change the MultipleJWKS creation minimum required JWK Sets from 2 to 1.

This is because it's convenient to use the Multi JWK Set implementation in scenarios where one or more JWK Set is required.

Closes: #87

[WayneWuQSR]

Hi @MicahParks. I have noticed a comment related to this change on the following lines:

1. keyfunc/multiple.go

   Line 11 in 276c2ca

   var ErrMultipleJWKSSize = errors.New("multiple JWKS must have two or more remote JWK Set resources")

2. keyfunc/multiple.go

   Line 19 in 276c2ca

   // GetMultiple creates a new MultipleJWKS. A map of length two or more JWKS URLs to Options is required.

   
   which still indicate the minimum jwk sets to is 2.

[MicahParks]

Thanks. I'll change that tonight.

[MicahParks]

@WayneWuQSR please find the corrected comment and error text in v2.0.3.