Update index.rst

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Requirements

• Fill out this template to the extent possible so that this PR can be reviewed in a timely manner.
• Replace the bracketed text below with your own.
• All new code requires tests to ensure against regressions.

Description of the Change

[ Addition of a step in setting up the Excel wrapper]

Benefits

[ This action was required for me to get CoolProp to work on Excel 2019 for MacOSX. ]

Possible Drawbacks

[ Unknown ]

Verification Process

[ I tried locating the .dylib file elsewhere, providing the full path to it, but it only worked if I used the full path indicated in the rest of the installation guide. It did not work for me w/o the full path. It would be great if someone else could confirm on another mac.]

Applicable Issues

[ NA]

[CLAassistant]

All committers have signed the CLA.

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Thank you!