Fixed the license abbreviation

| Q               | A
| --------------- | ---
| Bug fix?        | no
| New feature?    | no
| BC breaks?      | no
| Deprecations?   | no
| Tests pass?     | yes
| Related tickets | 
| License         | GNU GPLv3

Uses the following pattern `GPLv<version>[+]` specified on the [GNU Coding Standards](https://www.gnu.org/prep/standards/html_node/_002d_002dversion.html#g_t_002d_002dversion).