UpdateSourceControl operation should model source control type as an enum

In https://github.com/Azure/azure-rest-api-specs/blob/67edb3f35a6b7342c8e48b4f0dc55099504eff50/specification/web/resource-manager/Microsoft.Web/stable/2018-02-01/ResourceProvider.json#L144 instead of a string it really should be an enum. At the very least the doc string for this param should be updated with the applicable values.

Open specification/web/resource-manager/Microsoft.Web/stable/2018-02-01/ResourceProvider.json at the UpdateSourceControl operation and inspect the source-control-type parameter. Done means the parameter is modeled as an enum rather than a string, or its documentation lists the applicable values, while keeping the specification valid.

Written by the indexing model from the issue text.