Declare a clean url segment for StaticCacheFullBuildTask

[GuySartorelli]

The documentation indicates that the intention was for StaticCacheFullBuildTask to have a simple URL segment declared.

silverstripe-staticpublishqueue/docs/en/building_the_cache.md

Lines 16 to 29 in 2f05ec1

	You can generate cache files for the entire site via the `StaticCacheFullBuildJob`.
	This can either be queued up from the QueuedJobs interface or via the task
	`dev/tasks/StaticCacheFullBuildTask`. This task also takes a parameter `?startAfter`
	which can delay the execution of the job. The parameter should be in HHMM format,
	e.g. to start after 2pm, pass `?startAfter=1400`. If it's already after the proposed
	time on the current day, it will push it to the next day.
	If you want to do a full site build on a cron, you should do so via the task. An
	example configuration would be:
	```bash
	# build the cache at 1am every day
	0 1 * * * www-data /path/to/sake dev/tasks/StaticCacheFullBuildTask
	```

That task should have private static $segment = 'StaticCacheFullBuildTask'; added in the next major release

Acceptance criteria

• The url segment for StaticCacheFullBuildTask is StaticCacheFullBuildTask
• The documentation reflects the new url segment