Backport PR #5704: Update nbconvertapp.py

A spelling mistake in comment corrected in the line 109

IPython/nbconvert/nbconvertapp.py

@@ -116,7 +116,7 b' class NbConvertApp(BaseIPythonApplication):'
         You can specify the export format with `--to`.
         Options include {0}
         
-        > ipython nbconvert --to latex mynotebook.ipnynb
+        > 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