DOC: more accurate wording in roadmap
This came up the other day in https://github.com/pandas-dev/pandas/pull/40651#issuecomment-820671958.  As I [originally](https://github.com/pandas-dev/pandas/pull/35208#issuecomment-656879885) said in the PR that put this inaccurate wording in, this overstates the degree to which there is consensus.