🐛 fixed README example

The example with the size() operator is bad: using operator[] already
changes the size of the object. The fix makes it clearer.

doc/examples/README.cpp

@@ -1,4 +1,5 @@
 #include <json.hpp>
+#include <iomanip> // for std::setw
 
 using json = nlohmann::json;
 
@@ -29,7 +30,8 @@ int main()
     j["new"]["key"]["value"] = {"another", "list"};
 
     // count elements
-    j["size"] = j.size();
+    auto s = j.size();
+    j["size"] = s;
 
     // pretty print with indent of 4 spaces
     std::cout << std::setw(4) << j << '\n';