fix typo in examples

doc/examples/end.cpp

@@ -8,7 +8,7 @@ int main()
     // create an array value
     json array = {1, 2, 3, 4, 5};
 
-    // get am iterator to one past the last element
+    // get an iterator to one past the last element
     json::iterator it = array.end();
 
     // decrement the iterator to point to the last element