Update README.md to replace hyphen with em dash

[Yanofly]

Replaced incorrectly used hyphen with proper em dash.

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[Manish-Giri]

Hi,

Thanks for this pull request (PR).

Unfortunately, we are marking this PR invalid for hacktoberfest. We believe it does not follow the recommended guidelines:

Quality Standards

In line with Hacktoberfest value #2 (Quantity is fun, Quality is key), we have provided examples of the quality standards we encourage. This applies mainly to beginners.

• PRs that are automated e.g. scripted opening PRs to remove whitespace / optimize images.
• PRs that are disruptive e.g. taking someone else's branch/commits and making a PR.
• PRs that are regarded by a project maintainer as a hindrance vs. helping.
• Something that's clearly an attempt to simply +1 your PR count for October.

Learn more at https://hacktoberfest.digitalocean.com/details