[lintstaged] Rename keyServer => keyserver

Summary: keyserver is one "word" so shouldn't be camelCase.

Test Plan: Made a change in /keyserver and made sure that the lintstaged steps ran as expected.

Reviewers: tomek, jacek, abosh, varun

Reviewed By: abosh, varun

Subscribers: ashoat, adrian

Differential Revision: https://phab.comm.dev/D4620