[cxx-interop] Fix incorrectly referring to structure instead of class (#738)

Author: phyominthu

documentation/cxx-interop/index.md

@@ -1610,7 +1610,7 @@ void createMountainRange() {
 Swift classes that are exposed to C++ become C++ classes in the
 generated header. Top-level classes are placed in the C++ `namespace`
 that represents the Swift module. The exposed initializers, methods and
-properties defined inside of the Swift structure become members of the
+properties defined inside of the Swift class become members of the
 C++ class.
 
 The C++ class that represents a Swift class is copyable and movable.