fix import type 2

Author: jowezarek

exercises/poisson_2d_I.ipynb

@@ -133,7 +133,7 @@
     "from utils import H1BoundaryProjector2D\n",
     "from psydac.linalg.basic import IdentityOperator\n",
     "\n",
-    "P_0     = H1BoundaryProjector2D_2(V, V_h.vector_space)\n",
+    "P_0     = H1BoundaryProjector2D(V, V_h.vector_space)\n",
     "\n",
     "I_0     = IdentityOperator(V_h.vector_space)\n",
     "P_Gamma = I_0 - P_0"