Fix typo and dead link in Dendrite docs (#1363) #648

Job	Run time
Dendrite: SQLite	6m 59s
Dendrite: Postgres	11m 56s
Synapse: Py 3.8, SQLite 3.31.1, Monolith	9m 20s
Synapse: Py 3.8, PG 12, Monolith	12m 11s
Synapse: Py 3.8, PG 12, Workers	19m 16s
Synapse: Py 3.10, PG 14, Monolith	17m 59s
Synapse: Py 3.10, PG 14, Workers	22m 59s
Synapse: debian-testing, Monolith	12m 0s
Synapse: debian-testing, Workers	19m 19s
	2h 11m 59s

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