fixed a typo (#1110)

Author: nodeluna

src/ftxui/dom/size.cpp

@@ -81,7 +81,7 @@ class Size : public Node {
 }  // namespace
 
 /// @brief Apply a constraint on the size of an element.
-/// @param direction Whether the WIDTH of the HEIGHT of the element must be
+/// @param direction Whether the WIDTH or the HEIGHT of the element must be
 ///                  constrained.
 /// @param constraint The type of constaint.
 /// @param value The value.