Update README.md

Author: rohanjha123

README.md

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 # textbook
-Textbook for Econ 148: Data Science for Economists at UC Berkeley. Can be viewed at `[www.econ148.org/textbook](www.econ148.org/textbook)`.
+Textbook for Econ 148: Data Science for Economists at UC Berkeley. Can be viewed at [`www.econ148.org/textbook`](www.econ148.org/textbook).
 
 Content is stored in the content folder. Order of textbook can be changed from `_toc.yml` file. All required packages are in `requirements.txt` (generated by running `update_reqs.ipynb`).