prerelease example in the spec has an invalid dash

[steveklabnik]

https://semver.org/#spec-item-9

1.0.0-x-y-z.–. has a dash that's not a hyphen as the last example, which is not actually allowed by teh spec.

[mk-pmb]

Also there's an U+2014 em dash (—) in item 10, build metadata.

[Nixinova]

emdash issue is tracked at #291

[Nixinova]

And the endash issue in item 9 is caused by the same problem.

[JohnTitor]

#394 resolved this, closing.