doc: hyphen range constraint without whitespace

[johnnychen94]

I was a bit surprised that the following doesn't work as expected at first sight:

package main

import (
	"github.com/Masterminds/semver/v3"
)

func main() {
	c, err := semver.NewConstraint("0.7.2-0.8.0")
	if err != nil {
		panic(err)
	}
	v, err := semver.NewVersion("0.7.25")
	if err != nil {
		panic(err)
	}
	println(c.Check(v))
}

then I realized that we need whitespace between:

-	c, err := semver.NewConstraint("0.7.2-0.8.0")
+ 	c, err := semver.NewConstraint("0.7.2 - 0.8.0")