docs(undergrad): add pointwise convergence (#15601)

We could link to the definition of the product topology but I think it makes more sense to point out this theorem stating that convergence for the product topology is pointwise convergence.

docs/undergrad.yaml

@@ -346,7 +346,7 @@ Single Variable Real Analysis:
     absolute vs conditional convergence of improper integrals: 'https://en.wikipedia.org/wiki/Conditional_convergence'
     comparison test for improper integrals: 'https://mathbooks.unl.edu/Calculus/sec-5-11-comparison.html'
   Sequences and series of functions:
-    pointwise convergence: 'https://en.wikipedia.org/wiki/Pointwise_convergence'
+    pointwise convergence: 'tendsto_pi_nhds'
     uniform convergence: 'tendsto_uniformly'
     normal convergence: 'https://en.wikipedia.org/wiki/Normal_convergence'
     continuity of the limit: 'continuous_of_uniform_approx_of_continuous'