fix: osc8-link doubled backslashes in format string

The osc8-link implementation and its test both had doubled
backslashes (\\ -> \\) in their format strings, producing two
literal backslashes at runtime instead of the single backslash
needed for the OSC 8 string terminator (ST = ESC \).

Fix: change \\ to \\ in both the implementation and test format
strings. The tangled .lisp files now have correct escaped
backslashes (\) producing one backslash in the runtime string.

Additionally clean up a patch artifact that left a stray backslash
before the opening quote.

org/modern-backend.org

@@ -215,7 +215,7 @@ and ~\\ for literal backslash.
 (test osc8-escape
   "OSC 8 hyperlink escape wraps text"
   (is (equal (cl-tty.backend::osc8-link "http://example.com" "click here")
-             (format nil "~C]8;;http://example.com~C\\click here~C]8;;~C\"
+             (format nil "~C]8;;http://example.com~C\click here~C]8;;~C\"
                      #\Esc #\Esc #\Esc #\Esc))))
 #+END_SRC
 
@@ -475,7 +475,7 @@ allows clickable text in terminals that support the protocol.
 #+BEGIN_SRC lisp :tangle ../src/backend/modern.lisp
 (defun osc8-link (url text)
   "Wrap TEXT in an OSC 8 hyperlink to URL."
-  (format nil "~C]8;;~A~C\\~A~C]8;;~C\\"
+  (format nil \"~C]8;;~A~C\\~A~C]8;;~C\\\"
           #\Esc url #\Esc text #\Esc #\Esc))
 #+END_SRC