config: source job: rename field 'datasets' to 'filesystems'

While filesystems is also not the right term (since it excludes ZVOLs),
we want to stay consistent with comments & terminology used in docs.

BREAK CONFIG

fixes #17

docs/content/configuration/map_filter_syntax.md

@@ -94,7 +94,7 @@ jobs:
 - name: pull_backup
   type: source
   ...
-  datasets: {
+  filesystems: {
     "zroot/var/db": "ok",
     "zroot/usr/home<": "ok",
     "zroot/usr/home/paranoid": "!",

docs/content/tutorial/_index.md

@@ -102,7 +102,7 @@ jobs:
   serve:
     type: stdinserver
     client_identity: backup-srv.example.com
-  datasets: {
+  filesystems: {
     "zroot/var/db": "ok",
     "zroot/usr/home<": "ok",
     "zroot/usr/home/paranoid": "!",