fix typo in examples

doc/examples/begin.cpp

@@ -8,7 +8,7 @@ int main()
     // create an array value
     json array = {1, 2, 3, 4, 5};
 
-    // get am iterator to the first element
+    // get an iterator to the first element
     json::iterator it = array.begin();
 
     // serialize the element that the iterator points to