docs: quote NaN in stats/incr/nansumprod JSDoc summary

The JSDoc summary in `lib/main.js` wrote `NaN` unquoted, making it the only
documentation surface in the package to do so: `README.md`, `lib/index.js`,
`docs/repl.txt` and `docs/types/index.d.ts` all already backtick it. 69 of the
72 `nan*` members (95.8%) backtick `NaN` in their JSDoc summary, so this brings
the file in line with both its siblings and the rest of its own package.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FFCQ8XaJ5NUDv9oDpHF2Vj

Author: claude

lib/node_modules/@stdlib/stats/incr/nansumprod/lib/main.js

@@ -27,7 +27,7 @@ var isnan = require( '@stdlib/math/base/assert/is-nan' );
 // MAIN //
 
 /**
-* Returns an accumulator function which incrementally computes a sum of products, ignoring NaN values.
+* Returns an accumulator function which incrementally computes a sum of products, ignoring `NaN` values.
 *
 * @returns {Function} accumulator function
 *