Update docs/src/custom_workspaces.md

docs/src/custom_workspaces.md

@@ -6,7 +6,7 @@ The Poisson equation is a fundamental partial differential equation (PDE) common
 In a 2D Cartesian domain, it can be expressed as:
 
 ```math
-\nabla u(x, y) = f(x, y)
+\nabla^2 u(x, y) = f(x, y)
 ```
 
 Here, $u(x, y)$ represents the potential function we seek, while $f(x, y)$ is a known function (the "source term") that indicates the distribution of sources or sinks within the domain.