docs: use correct PDF reference link in stats/base/dists/signrank/pdf

PR-URL: #10303

Reviewed-by: Philipp Burckhardt <pburckhardt@outlook.com>

Author: shubham220420

lib/node_modules/@stdlib/stats/base/dists/signrank/pdf/README.md

@@ -235,7 +235,7 @@ int main( void ) {
 
 <section class="links">
 
-[pdf]: https://en.wikipedia.org/wiki/Cumulative_distribution_function
+[pdf]: https://en.wikipedia.org/wiki/Probability_density_function
 
 </section>