bpo-33641: Convert RFC references into links. (GH-7103)

85% of them are already links.
diff --git a/Doc/library/binascii.rst b/Doc/library/binascii.rst
index 4d3d0e0..a4efef8 100644
--- a/Doc/library/binascii.rst
+++ b/Doc/library/binascii.rst
@@ -80,7 +80,7 @@
    *quotetabs* is present and true, all tabs and spaces will be encoded.   If the
    optional argument *istext* is present and true, newlines are not encoded but
    trailing whitespace will be encoded. If the optional argument *header* is
-   present and true, spaces will be encoded as underscores per RFC1522. If the
+   present and true, spaces will be encoded as underscores per :rfc:`1522`. If the
    optional argument *header* is present and false, newline characters will be
    encoded as well; otherwise linefeed conversion might corrupt the binary data
    stream.