Update demo.md

articles/demo.md

@@ -148,7 +148,7 @@ $\lVert \boldsymbol{X}_i - \boldsymbol{S}_j \rVert^2$
 
 $$ f(x,y,z) = 3y^2z \left( 3+\frac{7x+5}{1+y^2} \right) $$
 
-$$ \left. \frac{{\rm d}u}{{\rm d}x} \right| _{x=0} $$
+<!-- $$ \left. \frac{{\rm d}u}{{\rm d} x} \right| _{x=0} $$ -->
 
 分数