step down toumorokoshi to approver (open-telemetry#901)

As discussed in the SIG, toumorokoshi is stepping down
as a maintainer.

Co-authored-by: alrex <aboten@lightstep.com>

Author: toumorokoshi

README.md

@@ -110,14 +110,14 @@ Approvers ([@open-telemetry/python-approvers](https://github.com/orgs/open-telem
 - [Hector Hernandez](https://github.com/hectorhdzg), Microsoft
 - [Mauricio Vásquez](https://github.com/mauriciovasquezbernal), Kinvolk
 - [Reiley Yang](https://github.com/reyang), Microsoft
+- [Yusuke Tsutsumi](https://github.com/toumorokoshi), Google
 
 *Find more about the approver role in [community repository](https://github.com/open-telemetry/community/blob/master/community-membership.md#approver).*
 
 Maintainers ([@open-telemetry/python-maintainers](https://github.com/orgs/open-telemetry/teams/python-maintainers)):
 
 - [Alex Boten](https://github.com/codeboten), Lightstep
 - [Leighton Chen](https://github.com/lzchen), Microsoft
-- [Yusuke Tsutsumi](https://github.com/toumorokoshi), Zillow Group
 
 *Find more about the maintainer role in [community repository](https://github.com/open-telemetry/community/blob/master/community-membership.md#maintainer).*