Fix the doller sign replacement regex issue on the car_sales Price column

[chanakaHetti]

I found an error while executing the code as bellow screenshot

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Solution:
In this solution I fixed the doller sign replacement regex issue on the car_sales Price column on introduction-to-pandas.ipynb file

I followed two steps to fix this,

1. Convert the Price column to string type
   car_sales["Price"] = car_sales["Price"].astype(str)

2. Use str.replace() to remove $, commas, and periods ans convert to the integer
   car_sales["Price"] = car_sales["Price"].str.replace(r'[\$\,\.]', '', regex=True)

I think from this changes this 87 issue will be solved.
#87

Thank you. Enjoy!