Update README.rst

The example in readme cannot work now, have to use 'GOOGL' after the split in January. GOOG only stands for C share, and yahoo.com does not have historical information of 'GOOG' anymore, use 'GOOGL' instead.

still other queries for 'GOOG' works NOW.(but only till June 20,2014 I believe, then NASDAQ-100 will remove 'GOOG' and use 'GOOGL' after then)