Remove duplicate "Reference Guide" in optparse.rst. (GH-10372)

diff --git a/Doc/library/optparse.rst b/Doc/library/optparse.rst
index 3afc77b..16f1cf0 100644
--- a/Doc/library/optparse.rst
+++ b/Doc/library/optparse.rst
@@ -415,7 +415,7 @@
 ``"callback"``
    call a specified function
 
-These are covered in section :ref:`optparse-reference-guide`, Reference Guide
+These are covered in section :ref:`optparse-reference-guide`,
 and section :ref:`optparse-option-callbacks`.