chore: update langfuse description (#15136)

web/i18n/en-US/app.ts

     },
     langfuse: {
       title: 'Langfuse',
-      description: 'Traces, evals, prompt management and metrics to debug and improve your LLM application.',
+      description: 'Open-source LLM observability, evaluation, prompt management and metrics to debug and improve your LLM application.',
     },
     opik: {
       title: 'Opik',