docs: rename variable

Author: swsoyee

README.md

@@ -31,9 +31,9 @@ If you're using Node / Webpack / Rollup / Browserify, etc, simply require the la
 
 ```javascript
 var hljs = require('highlight.js');
-var hljsRobotsTxt = require('highlightjs-spl');
+var hljsSpl = require('highlightjs-spl');
 
-hljs.registerLanguage("spl", hljsRobotsTxt);
+hljs.registerLanguage("spl", hljsSpl);
 hljs.initHighlightingOnLoad();
 ```