docs: remove reference to /etc/river/init

We used to look in /etc/river/init if no init at ~/.config/river/init
or $XDG_CONFIG_HOME/river/init was found but this feature was
removed. It seems that we forgot to remove this mention of the old
behavior however.

 doc/riverctl.1.scd | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/doc/riverctl.1.scd b/doc/riverctl.1.scd
index ea43930..7955e36 100644
--- a/doc/riverctl.1.scd
+++ b/doc/riverctl.1.scd
@@ -355,7 +355,9 @@ Bind Super+Return in normal mode to spawn a *foot*(1) terminal:
 
 	riverctl map normal Mod4 Return spawn 'foot --app-id=foobar'
 
-See also the example init script at /etc/river/init.
+Bind Super+Shift+J to swap the focused view with the next visible view:
+
+	riverctl map normal Mod4+Shift J swap next
 
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