fix: correct median formula in kumaraswamy distribution

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Author: Krishna-Sharma-g

lib/node_modules/@stdlib/stats/base/dists/kumaraswamy/median/src/main.c

@@ -39,5 +39,5 @@ double stdlib_base_dists_kumaraswamy_median( const double a, const double b ) {
 	) {
 		return 0.0/0.0; // NaN
 	}
-	return 1.0 - stdlib_base_pow( 0.5, 1.0 / b );
+	return stdlib_base_pow( 1.0 - stdlib_base_pow( 2.0, -1.0 / b ), 1.0 / a );
 }