Update nbconvertapp.py

A spelling mistake in comment corrected in the line 109
Pygment now adds a language-class to the highlighted html so that
postprocessors can use it to identify the code block and the language
of the code inside.
Adjusted "rawcell" toolbar preset to make us of the new "register_callback"
optional parameter.
Merge pull request #5704 from dexterdev/patch-1

Update nbconvertapp.py
Merge pull request #4682 from dbarbeau/syntax-highlight

Javascript hightlighting for NBConvert html output
Fix a NameError in IPython.parallel
Merge pull request #5713 from takluyver/parallel-dependency-nameerror

Fix a NameError in IPython.parallel
Merge pull request #5717 from AlbertHilb/RawCell

"rawcell" toolbar preset adjustment