Corrected the em dash and en dash symbols in plastexit.py
Corrected the em dash and en dash symbols in plastexit.py

mitocw/latex2edx,jefrench/latex2edx,jefrench/latex2edx,evheubel/latex2edx,evheubel/latex2edx,evheubel/latex2edx,mitocw/latex2edx,jefrench/latex2edx,mitocw/latex2edx