Update README.md

Author: LesterLyu

README.md

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 Aim to be the fastest and the most reliable excel formula parser in javascript. Using **LL(1)** parser.
 ### [Demo](https://lesterlyu.github.io/demo/fast-formula-parser)
-### [Documentation](https://lesterlyu.github.io/fast-formula-parser/)
+### [Documentation](https://lesterlyu.github.io/fast-formula-parser/index.html)
 ### [Grammar Diagram](https://lesterlyu.github.io/fast-formula-parser/generated_diagrams.html)
 ### Background