orgs: update create mutation

[leonore]

running src orgs create -name X was failing with

GraphQL errors: {
  "locations": [
    {
      "column": 3,
      "line": 5
    }
  ],
  "message": "Cannot query field \"createOrg\" on type \"Mutation\". Did you mean \"createUser\"?"
}

the mutation name should be createOrganization

Test plan

SRC_ENDPOINT="https://sourcegraph.test:3443" SRC_ACCESS_TOKEN=${SG_TEST_TOKEN} go run ./cmd/src orgs create -name ce-team
Organization "ce-team" created.

[eseliger]

😬