Update README.md

README.md

@@ -54,7 +54,7 @@ a000
 ```
 i.e.
 
-![](http://latex.codecogs.com/gif.latex?\{density contrast}=a_{000}+a_{100}x+a_{010}y+a_{001}z+a_{200}x^2+a_{020}y^2+a_{002}z^2+a_{101}xz+a_{011}yz+a_{110}xy+a_{003}z^3+a_{012}yz^2+a_{021}y^2z+a_{030}y^3+a_{102}xz^2+a_{111}xyz+a_{120}xy^2+a_{201}x^2z+a_{210}x^2y+a_{300}x^3)
+![](http://latex.codecogs.com/gif.latex?\\rho=a_{000}+a_{100}x+a_{010}y+a_{001}z+a_{200}x^2+a_{020}y^2+a_{002}z^2+a_{101}xz+a_{011}yz+a_{110}xy+a_{003}z^3+a_{012}yz^2+a_{021}y^2z+a_{030}y^3+a_{102}xz^2+a_{111}xyz+a_{120}xy^2+a_{201}x^2z+a_{210}x^2y+a_{300}x^3)
 
 In the model file, all polyhedrons, facets of a single polyhedron,and corners of a single facet are numbered from zero. The units for x, y, z are kilometer, and the unit for density contrast is kilogram per cubic meter.