api docs: Refactor get_single_user to use get_user_by_id.
api docs: Refactor get_single_user to use get_user_by_id.

This refactors get_single_user to use get_user_by_id instead of
call_endpoint.  Doing so is only possible now that we've upgraded
python-zulip-api to a version with the new function.
brainwane/zulip,eeshangarg/zulip,rht/zulip,showell/zulip,kou/zulip,eeshangarg/zulip,shubhamdhama/zulip,timabbott/zulip,kou/zulip,zulip/zulip,punchagan/zulip,synicalsyntax/zulip,andersk/zulip,shubhamdhama/zulip,rht/zulip,andersk/zulip,eeshangarg/zulip,eeshangarg/zulip,punchagan/zulip,synicalsyntax/zulip,showell/zulip,brainwane/zulip,brainwane/zulip,zulip/zulip,brainwane/zulip,timabbott/zulip,timabbott/zulip,hackerkid/zulip,andersk/zulip,punchagan/zulip,zulip/zulip,rht/zulip,brainwane/zulip,shubhamdhama/zulip,punchagan/zulip,zulip/zulip,showell/zulip,eeshangarg/zulip,kou/zulip,rht/zulip,kou/zulip,zulip/zulip,synicalsyntax/zulip,shubhamdhama/zulip,showell/zulip,timabbott/zulip,punchagan/zulip,zulip/zulip,rht/zulip,kou/zulip,showell/zulip,eeshangarg/zulip,rht/zulip,andersk/zulip,hackerkid/zulip,brainwane/zulip,timabbott/zulip,timabbott/zulip,showell/zulip,kou/zulip,synicalsyntax/zulip,hackerkid/zulip,hackerkid/zulip,hackerkid/zulip,synicalsyntax/zulip,timabbott/zulip,andersk/zulip,hackerkid/zulip,shubhamdhama/zulip,zulip/zulip,hackerkid/zulip,shubhamdhama/zulip,eeshangarg/zulip,shubhamdhama/zulip,punchagan/zulip,andersk/zulip,synicalsyntax/zulip,showell/zulip,andersk/zulip,punchagan/zulip,kou/zulip,rht/zulip,brainwane/zulip,synicalsyntax/zulip