docs(readme): fix default preview background job execution time

It is set to 300 seconds (5 minutes) by default in the config service
class.

Signed-off-by: Richard Steinmetz <richard@steinmetz.cloud>

Author: st3iny

README.md

@@ -123,7 +123,7 @@ Set to true to disable the background job that generates previews by default wit
 configure a manual system cron job. It is recommended to disable the default background job in case
 a custom system cron entry with `occ preview:pre-generate` is configured (set this config to true).
 
-#### `occ config:app:set --value=600 --type=integer previewgenerator job_max_execution_time`
+#### `occ config:app:set --value=300 --type=integer previewgenerator job_max_execution_time`
 Limits the maximum execution time in seconds of the preview background job. (A value of zero means
 unlimited.)