resolve example failure issue.

Signed-off-by: Harsh <[email protected]>

Author: hrshya

lib/node_modules/@stdlib/math/base/special/polygamma/README.md

@@ -74,7 +74,7 @@ v = polygamma( 2, -1.0 );
 If `x` on the other hand is a negative even `integer`, the function returns `NaN`.
 
 ```javascript
-v = polygamma( 2, -4.0 );
+var v = polygamma( 2, -4.0 );
 // returns NaN
 
 v = polygamma( 2, -2.0 );