fix typo in python3_version

The function was using an undefined variable to show the version of
python3 being used.

Change-Id: Ibc956975d620ed5174de8823f9c202a680c56aaf
Signed-off-by: Doug Hellmann <doug@doughellmann.com>
diff --git a/functions-common b/functions-common
index b1b0995..25e28bd 100644
--- a/functions-common
+++ b/functions-common
@@ -2128,7 +2128,7 @@
 function python3_version {
     local python3_version
     python3_version=$(_get_python_version python3)
-    echo "python${python_version}"
+    echo "python${python3_version}"
 }