zmqpp: Use language-specific heredocs

Author: issyl0

Formula/z/zmqpp.rb

@@ -37,13 +37,13 @@ def install
   end
 
   test do
-    (testpath/"test.cpp").write <<~EOS
+    (testpath/"test.cpp").write <<~CPP
       #include <zmqpp/zmqpp.hpp>
       int main() {
         zmqpp::frame frame;
         return 0;
       }
-    EOS
+    CPP
     system ENV.cxx, "test.cpp", "-L#{lib}", "-lzmqpp", "-o", "test", "-std=c++11"
     system "./test"
   end