docs: update example in variancetk function for clarity

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---

Author: gururaj1512

lib/node_modules/@stdlib/stats/base/variancetk/lib/accessors.js

@@ -37,10 +37,10 @@
 * var toAccessorArray = require( '@stdlib/array/base/to-accessor-array' );
 * var arraylike2object = require( '@stdlib/array/base/arraylike2object' );
 *
-* var x = toAccessorArray( [ 1.0, -2.0, 2.0 ] );
+* var x = toAccessorArray( [ 2.0, 1.0, 2.0, -2.0, -2.0, 2.0, 3.0, 4.0 ] );
 *
-* var v = variancetk( x.length, 1, arraylike2object( x ), 1, 0 );
-* // returns ~4.3333
+* var v = variancetk( 4, 1, arraylike2object( x ), 2, 1 );
+* // returns 6.25
 */
 function variancetk( N, correction, x, strideX, offsetX ) {
 	var xbuf;
@@ -52,17 +52,13 @@ function variancetk( N, correction, x, strideX, offsetX ) {
 	var n;
 	var i;
 
-	n = N - correction;
-	if ( N <= 0 || n <= 0.0 ) {
-		return NaN;
-	}
-
 	// Cache reference to array data:
 	xbuf = x.data;
 
 	// Cache a reference to the element accessor:
 	get = x.accessors[ 0 ];
 
+	n = N - correction;
 	if ( N === 1 || strideX === 0 ) {
 		return get( xbuf, offsetX );
 	}