Replace "matrix-coturn" with "coturn"

Keep "matrix-coturn" on documentation as-is, since it is specified so with `coturn_identifier`.

Signed-off-by: Suguru Hirahara <did🔑z6MkvVZk1A3KBApWJXv2Ju4H14ErDfRGxh8zxdXSZ4vACDg5>

docs/self-building.md

@@ -31,7 +31,7 @@ Possibly outdated list of roles where self-building the Docker image is currentl
 - `matrix-client-hydrogen`
 - `cinny`
 - `matrix-registration`
-- `matrix-coturn`
+- `coturn`
 - `matrix-corporal`
 - `matrix-dimension`
 - `exim-relay`