avocado.utils.process: Fix stderr prefixing

Signed-off-by: NLucas Meneghel Rodrigues <lmr@redhat.com>

avocado/utils/process.py

@@ -153,7 +153,7 @@ class SubProcess(object):
             output_file = self.stdout_file
             lock = self.stdout_lock
         elif input_pipe == self.sp.stderr:
-            prefix = '[stdout] %s'
+            prefix = '[stderr] %s'
             output_file = self.stderr_file
             lock = self.stderr_lock