docs(sandboxes): use positional [bind_address:]port in IPv6 forward example

`openshell forward start` has no -b/--bind-addr option; the bind
address is passed positionally as [bind_address:]port. Update the prose
and command so the example parses (openshell forward start ::1:8000
my-sandbox).

Signed-off-by: Andrew White <andrewh@cdw.com>

Author: andrewwhitecdw

docs/sandboxes/manage-sandboxes.mdx

@@ -361,10 +361,10 @@ openshell forward start 8000 my-sandbox -d    # run in background
 
 OpenShell prints the local URL only after the forward listener is reachable. Background forwards must be tracked locally so `openshell forward list` and `openshell forward stop` can manage them.
 
-To bind the local side to an IPv6 loopback address, use `-b`/`--bind-addr`. IPv6 addresses are bracketed in both the SSH forward argument and the printed access URL:
+To bind the local side to an IPv6 loopback address, pass it positionally as `[bind_address:]port`. IPv6 addresses are bracketed in both the SSH forward argument and the printed access URL:
 
 ```shell
-openshell forward start 8000 my-sandbox -b ::1
+openshell forward start ::1:8000 my-sandbox
 # Local URL: http://[::1]:8000/
 ```