Adjust :unbind signature to match :bind

doc/help/commands.asciidoc

@@ -933,13 +933,15 @@ How many tabs to switch back.
 
 [[unbind]]
 === unbind
-Syntax: +:unbind 'key' ['mode']+
+Syntax: +:unbind [*--mode* 'mode'] 'key'+
 
 Unbind a keychain.
 
 ==== positional arguments
 * +'key'+: The keychain or special key (inside <...>) to unbind.
-* +'mode'+: A mode to unbind the key in (default: `normal`). See `:help bindings.commands` for the available modes.
+
+==== optional arguments
+* +*-m*+, +*--mode*+: A mode to unbind the key in (default: `normal`). See `:help bindings.commands` for the available modes.
 
 
 [[undo]]

qutebrowser/config/config.py

@@ -347,7 +347,7 @@ class ConfigCommands:
             raise cmdexc.CommandError("bind: {}".format(e))
 
     @cmdutils.register(instance='config-commands')
-    def unbind(self, key, mode='normal'):
+    def unbind(self, key, *, mode='normal'):
         """Unbind a keychain.
 
         Args:

tests/unit/config/test_config.py

@@ -572,7 +572,7 @@ class TestBindConfigCommand:
         Should show an error.
         """
         with pytest.raises(cmdexc.CommandError, match=expected):
-            commands.unbind(key, mode)
+            commands.unbind(key, mode=mode)
 
 
 class TestConfig: