typo: dump_api -> dumpapi in error message

Bug: n/a
Test: aidl --dumpapi
Change-Id: Ia7aa9f83a414f1590f1ee14237d35129e81f3b85
diff --git a/options.cpp b/options.cpp
index 1018146..9e5df3c 100644
--- a/options.cpp
+++ b/options.cpp
@@ -502,7 +502,7 @@
   }
   if (task_ == Options::Task::DUMP_API) {
     if (output_dir_.empty()) {
-      error_message_ << "--dump_api requires output directory. Use --out." << endl;
+      error_message_ << "--dumpapi requires output directory. Use --out." << endl;
       return;
     }
   }