docs: normalize example variable naming in @stdlib/stats/base/dists/chisquare/median

Replaced `var y` and `var k` with `var v` in the negative-input and NaN
`@example` blocks so the package matches the convention used by all
seven single-argument sibling distribution packages (`entropy`,
`kurtosis`, `mean`, `mode`, `skewness`, `stdev`, `variance`), where
`var v = <pkg>( ... );` appears uniformly across all four examples in
both the README and the TypeScript declaration file (87.5%
conformance). The package's own `lib/main.js` already uses `var v`
throughout, so the divergence was confined to `README.md` and
`docs/types/index.d.ts`.

Author: claude

lib/node_modules/@stdlib/stats/base/dists/chisquare/median/README.md

@@ -55,7 +55,7 @@ v = median( 2.0 );
 If provided `k < 0`, the function returns `NaN`.
 
 ```javascript
-var y = median( -1.0 );
+var v = median( -1.0 );
 // returns NaN
 ```
 

lib/node_modules/@stdlib/stats/base/dists/chisquare/median/docs/types/index.d.ts

@@ -37,11 +37,11 @@
 * // returns ~1.386
 *
 * @example
-* var k = median( -0.2 );
+* var v = median( -0.2 );
 * // returns NaN
 *
 * @example
-* var k = median( NaN );
+* var v = median( NaN );
 * // returns NaN
 */
 declare function median( k: number ): number;