fix: correct mathematical notation and add return statement in README

- Changed printf format from 'logf(x;k)' to 'ln(f(x;k))' for consistency
- Added missing 'return 0;' statement in C example

Author: Planeshifter

lib/node_modules/@stdlib/stats/base/dists/chi/logpdf/README.md

@@ -222,8 +222,9 @@ int main( void ) {
         x = random_uniform( 0.0, 10.0 );
         k = random_uniform( 0.1, 10.0 );
         y = stdlib_base_dists_chi_logpdf( x, k );
-        printf( "x: %lf, k: %lf, logf(x;k): %lf\n", x, k, y );
+        printf( "x: %lf, k: %lf, ln(f(x;k)): %lf\n", x, k, y );
     }
+    return 0;
 }
 ```