[cxx-interop] Fix incorrectly referring to "structure" instead of "class"

[phyominthu]

Fix incorrectly referring to "structure" instead of "class" in a section of C++ interoperability documentation.

Motivation:

Under Using Swift Classes in C++, the documentation is incorrectly referring to the Swift "structure" instead of "class".

Modifications:

Replace the Swift "structure" with "class".

Result:

Before:

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 C++ class.

After:

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 class become members of the C++ class.

[shahmishal]

cc: @egorzhdan

[egorzhdan]

@swift-ci please test