rename doc group to docs

Author: Jiaqi Liu

.travis.yml

@@ -19,7 +19,7 @@ env:
     - GROUP=js/base
     - GROUP=js/services
     - GROUP=js/tree
-    - GROUP=doc
+    - GROUP=docs
 
 before_install:
     - pip install --upgrade pip
@@ -48,7 +48,7 @@ install:
 script:
     - 'if [[ $GROUP == js* ]]; then travis_retry python -m notebook.jstest ${GROUP:3}; fi'
     - 'if [[ $GROUP == python ]]; then nosetests -v --with-coverage --cover-package=notebook notebook; fi'
-    - 'if [[ $GROUP == doc ]]; then make -C docs/ html; fi'
+    - 'if [[ $GROUP == docs ]]; then make -C docs/ html; fi'
 
 matrix:
     include: