Fix defadvice warning

- dtrt-indent.el:223:9: Warning: defvar
  ‘dtrt-indent-language-syntax-table’ docstring contains control char
  #x01 (position 629)
- dtrt-indent.el:223:9: Warning: defvar
  ‘dtrt-indent-language-syntax-table’ docstring contains control char
  #x02 (position 671)

dtrt-indent.el

@@ -1124,19 +1124,17 @@ Indentation offset set with file variable; not adjusted")
 ;;-----------------------------------------------------------------
 ;; Installation
 
-(defadvice hack-one-local-variable
-    (before dtrt-indent-advise-hack-one-local-variable activate)
-  "Adviced by dtrt-indent.
-
-Disable dtrt-indent if offset explicitly set."
+(defun dtrt-indent-advise-hack-one-local-variable (var _val &rest _)
+  "Advice for `hack-one-local-variable' to disable dtrt-indent when necessary.
+VAR corresponds to the first argument of `hack-one-local-variable'."
   (cond
-   ((eql (nth 2 (dtrt-indent--search-hook-mapping major-mode))
-         (ad-get-arg 0))
+   ((eql (nth 2 (dtrt-indent--search-hook-mapping major-mode)) var)
     (setq dtrt-indent-explicit-offset t))
-   ((eql 'indent-tabs-mode
-         (ad-get-arg 0))
+   ((eql 'indent-tabs-mode var)
     (setq dtrt-indent-explicit-tab-mode t))))
 
+(advice-add 'hack-one-local-variable :before #'dtrt-indent-advise-hack-one-local-variable)
+
 (autoload 'dtrt-indent-diagnosis "dtrt-indent-diag"
   "Guess indentation for the current buffer and output diagnostics."
   t)