fix: use correct name of variable

Author: dennis-rall

README.md

@@ -47,7 +47,7 @@ JavaScript / TypeScript
 ```ts
 import { Sandbox } from '@e2b/code-interpreter'
 
-const sandbox = await Sandbox.create()
+const sbx = await Sandbox.create()
 await sbx.runCode('x = 1')
 
 const execution = await sbx.runCode('x+=1; x')