docs(emqx_connector): Remove mentions of cluster_singleton

[ieQu1]

Fixes: #8339

[coveralls]

Pull Request Test Coverage Report for Build 2609124104

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 35 unchanged lines in 11 files lost coverage.
• Overall coverage decreased (-0.08%) to 78.827%

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Files with Coverage Reduction	New Missed Lines	%
apps/emqx_conf/src/emqx_conf_schema.erl	1	92.31%
apps/emqx_gateway/src/mqttsn/emqx_sn_channel.erl	1	72.92%
apps/emqx_gateway/src/mqttsn/emqx_sn_frame.erl	1	63.48%
apps/emqx/src/emqx_cm.erl	1	88.99%
apps/emqx/src/emqx_router_utils.erl	1	58.14%
apps/emqx/src/emqx_stats.erl	1	93.33%
apps/emqx/src/emqx_ws_connection.erl	1	76.73%
apps/emqx/src/emqx_connection.erl	2	84.7%
apps/emqx/src/emqx_channel.erl	4	85.53%
apps/emqx/src/emqx_sys_mon.erl	6	83.54%

Totals
Change from base Build 2605144224:	-0.08%
Covered Lines:	20811
Relevant Lines:	26401

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💛 - Coveralls