[cpp doc improvement] cmake fetching

[hsdk123]

The current docs recommend

include(FetchContent)
FetchContent_Declare(
 spine-runtimes
 GIT_REPOSITORY https://github.com/esotericsoftware/spine-runtimes.git
 GIT_TAG 4.2
 GIT_SHALLOW TRUE
)
FetchContent_MakeAvailable(spine-runtimes)
FetchContent_GetProperties(spine-runtimes)
if(NOT spine-runtimes_POPULATED)
   FetchContent_Populate(spine-runtimes)
endif()
add_subdirectory(${spine-runtimes_SOURCE_DIR}/spine-c ${CMAKE_BINARY_DIR}/spine-runtimes)

But

• spine-c is mentioned, when it should be spine-cpp
• the implementation is against cmake standards

I'd like to propose:

FetchContent_Declare(
 spine-runtimes
 GIT_REPOSITORY https://github.com/esotericsoftware/spine-runtimes.git
 GIT_TAG 4.2
 GIT_SHALLOW TRUE
 SOURCE_SUBDIR spine-cpp
)
FetchContent_MakeAvailable(spine-runtimes)

Which achieves the same goal but without the above cons.
I'd send in a PR, but I don't notice a repo for one.