services: 'service-parameters' becomes 'service-value'.

* gnu/services.scm (<service>)[parameters]: Rename to...
[value]: ... this.
Change calls to 'service-parameters' to 'service-value'.
* gnu/system.scm, gnu/tests/base.scm,
guix/scripts/system.scm, tests/services.scm: Likewise.
* doc/guix.texi (Service Reference): Adjust accordingly.