Change OPEN_AI_TOKEN to OPENAI_API_KEY (#203)

Author: kpgalligan

examples/src/main/kotlin/ai/koog/agents/example/ApiKeyService.kt

@@ -2,7 +2,7 @@ package ai.koog.agents.example
 
 internal object ApiKeyService {
     val openAIApiKey: String
-        get() = System.getenv("OPEN_AI_TOKEN") ?: throw IllegalArgumentException("OPENAI_API_KEY env is not set")
+        get() = System.getenv("OPENAI_API_KEY") ?: throw IllegalArgumentException("OPENAI_API_KEY env is not set")
 
     val anthropicApiKey: String
         get() = System.getenv("ANTHROPIC_API_KEY") ?: throw IllegalArgumentException("ANTHROPIC_API_KEY env is not set")