fix typo in nbconvert help

Latex template is 'base', not 'basic'
cf. https://github.com/ipython/ipython/tree/master/IPython/nbconvert/templates/latex

IPython/nbconvert/nbconvertapp.py

@@ -108,7 +108,7 b' class NbConvertApp(BaseIPythonApplication):'
         > ipython nbconvert --to latex mynotebook.ipynb
 
         Both HTML and LaTeX support multiple output templates. LaTeX includes
-        'basic', 'book', and 'article'.  HTML includes 'basic' and 'full'.  You 
+        'base', 'book', and 'article'.  HTML includes 'basic' and 'full'.  You 
         can specify the flavor of the format used.
 
         > ipython nbconvert --to html --template basic mynotebook.ipynb