change characteristic operator to capital C to avoid clash with builtin \char

src/math/mathop.pysty

@@ -161,7 +161,7 @@ __END_OPTIONS_X__
 \DeclareSimpleMathOperator{Br}
 \DeclareSimpleMathOperator{EXT}
 \DeclareSimpleMathOperator{Ind}
-\DeclareSimpleMathOperator{char}
+\DeclareSimpleMathOperator{Char}
 \DeclareSimpleMathOperator{res}
 \DeclareSimpleMathOperator{inf}
 \DeclareSimpleMathOperator{cov}