Fix misspellings of CompletionsMenu in docs

Author: MrMino

prompt_toolkit/layout/containers.py

@@ -751,7 +751,7 @@ class FloatContainer(Container):
                        floats=[
                            Float(xcursor=True,
                                 ycursor=True,
-                                content=CompletionMenu(...))
+                                content=CompletionsMenu(...))
                        ])
 
     :param z_index: (int or None) When specified, this can be used to bring

prompt_toolkit/layout/scrollable_pane.py

@@ -29,7 +29,7 @@ class ScrollablePane(Container):
 
         If you want to display a completion menu for widgets in this
         `ScrollablePane`, then it's still a good practice to use a
-        `FloatContainer` with a `CompletionMenu` in a `Float` at the top-level
+        `FloatContainer` with a `CompletionsMenu` in a `Float` at the top-level
         of the layout hierarchy, rather then nesting a `FloatContainer` in this
         `ScrollablePane`. (Otherwise, it's possible that the completion menu
         is clipped.)