Repaired README example for point clouds

[amock]

Hi!

I tried your library for rendering a point cloud and used parts of your README as an example. The C++ example seemed to be missing an include and a semicolon. Here is the PR to fix that.

Thanks for publishing this great library -- Alex

[amock]

And BTW: I was able to very easily integrate your library into my cmake project by using the FetchContent functionality of cmake. I am not sure if it's written somewhere in the Docs. Here is my CMakeLists.txt:

cmake_minimum_required(VERSION 3.11)
project(polyscope_test)

include(FetchContent)

FetchContent_Declare(
   polyscope
  GIT_REPOSITORY https://github.com/nmwsharp/polyscope.git
  GIT_TAG        master
)

FetchContent_MakeAvailable(polyscope)

add_executable(polyscope_test
    src/polyscope_test.cpp)
target_link_libraries(polyscope_test 
    polyscope)

[nmwsharp]

thank you for the fix!

[nmwsharp]

I am also integrating the FetchContent example you gave into the docs, thank you!