Feat: change frac to dfrac for fractions

mapytex/calculus/API/expression.py

@@ -74,10 +74,10 @@ class Expression(object):
         2 + 3 \\times 4
         >>> e = Expression.from_str("2+3/4")
         >>> print(e)
-        2 + \\frac{3}{4}
+        2 + \\dfrac{3}{4}
         >>> es = e.simplify()
         >>> print(es)
-        \\frac{11}{4}
+        \\dfrac{11}{4}
         >>> Expression.set_render('txt')
         """
         from .tokens.token import Token