trivial: Rename docs/ => doc/ in .zuul.yaml

This renames docs/ => doc/ in .zuul.yaml ignore-files
stanza as Pegleg documentation directory was renamed to doc/

Change-Id: Ice37901d3a4fbb1f5978a847a83320873a451e82

.zuul.yaml

@@ -73,7 +73,7 @@
     run: tools/gate/playbooks/docker-image-build.yaml
     nodeset: airship-pegleg-single-node
     irrelevant-files:
-      - '^docs/.*'
+      - '^doc/.*'
     vars:
       publish: false
       tags:
@@ -88,7 +88,7 @@
     secrets:
       - airship_pegleg_quay_creds
     irrelevant-files:
-      - '^docs/.*'
+      - '^doc/.*'
     vars:
       publish: true
       tags: