Fix typo in poisson 1D neumann+dirichlet demo (#470)

Author: Saransh-cpp

.gitignore

@@ -6,4 +6,7 @@ __pycache__/
 docs/_build/
 
 # Jupyter notebook
-.ipynb_checkpoints
+.ipynb_checkpoints
+
+# VSCode
+.vscode/

docs/demos/poisson.1d.neumanndirichlet.rst

@@ -81,7 +81,7 @@ Then, the Dirichlet boundary condition is
 
 .. code-block:: python
 
-    bc_l = dde.DirichletBC(geom, func, boundary_1)
+    bc_l = dde.DirichletBC(geom, func, boundary_l)
 
 For Neumann boundary condition, rather than define a function to return the value of :math:`u(x)` on the boundary, we use a lambda function and the Neumann boundary condition is defined