Update opentelemetry-propagator-google_cloud_platform.gemspec

Author: ollym

propagator/google_cloud_platform/opentelemetry-propagator-google_cloud_platform.gemspec

@@ -40,5 +40,5 @@ Gem::Specification.new do |spec|
     spec.metadata['documentation_uri'] = "https://rubydoc.info/gems/#{spec.name}/#{spec.version}"
   end
 
-  spec.post_install_message = File.read(File.expand_path('../../POST_INSTALL_MESSAGE', __dir__))
+  spec.post_install_message = File.read(File.expand_path('../../gemspecs/POST_INSTALL_MESSAGE', __dir__))
 end