fix rendering part 2

pina/problem/zoo/helmholtz.py

@@ -53,6 +53,9 @@ class HelmholtzProblem(SpatialProblem):
         **Original reference**: Si, Chenhao, et al. *Complex Physics-Informed
         Neural Network.* arXiv preprint arXiv:2502.04917 (2025).
         DOI: `arXiv:2502.04917 <https://arxiv.org/abs/2502.04917>`_.
+    
+    :Example:
+        >>> problem = HelmholtzProblem()
     """
 
     output_variables = ["u"]