Update poisson.1d.dirichletrobin.rst (#1499)

Author: haruotsu

docs/demos/pinn_forward/poisson.1d.dirichletrobin.rst

@@ -75,7 +75,7 @@ Then, the Dirichlet boundary condition is
 
 .. code-block:: python
 
-    bc_l = dde.icbc.DirichletBC(geom, func, boundary_1)
+    bc_l = dde.icbc.DirichletBC(geom, func, boundary_l)
 
 For Robin boundary condition, rather than define a function to return the value of :math:`u(x)` on the boundary, we use a lambda function that maps ``x`` and ``y`` to ``y``, where x is the input and y is the output.  Then Robin boundary condition is defined