chore: fix 2

Author: Girish-Garg

lib/node_modules/@stdlib/stats/incr/nankurtosis/README.md

@@ -109,10 +109,10 @@ kurtosis = accumulator( 2.0 );
 kurtosis = accumulator( -4.0 );
 // returns null
 
-kurtosis = accumulator( NaN );
+kurtosis = accumulator( -4.0 );
 // returns -6.0
 
-kurtosis = accumulator( -4.0 );
+kurtosis = accumulator( NaN );
 // returns -6.0
 ```