[Validator] Rename requireHyphens to requireHyphen for Issn constraint

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        | 

While writing the documentation for the new Issn validator submitted yesterday at https://github.com/symfony/symfony/pull/7756, I've realized that the option ```requireHyphens``` should be renamed to ```requireHyphen``` because only one hyphen is allowed in the ISSN value.