[glibmm] stringutils: Don't use @retval here either

[Daniel Boles <dboles src gnome org>]

commit d35f936e7aa87a822a380d24f63339f81608f138
Author: Daniel Boles <dboles src gnome org>
Date:   Mon Dec 17 18:28:59 2018 +0000

    stringutils: Don't use @retval here either
    
    as pointed out by Kjell:
    https://gitlab.gnome.org/GNOME/gtkmm/merge_requests/9#note_389406

 glib/glibmm/stringutils.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/glib/glibmm/stringutils.h b/glib/glibmm/stringutils.h
index e6659e2b..466a9b6e 100644
--- a/glib/glibmm/stringutils.h
+++ b/glib/glibmm/stringutils.h
@@ -82,7 +82,7 @@ double strtod(const std::string& str);
  *
  * @param str The string to convert to a numeric value.
  * @param start_index The index of the first character that should be used in the conversion.
- * @retval end_index The index of the character after the last character used in the conversion.
+ * @param[out] end_index The index of the character after the last character used in the conversion.
  * @return The <tt>double</tt> value.
  * @throw std::out_of_range    Thrown if @a start_index is out of range.
  * @throw std::overflow_error  Thrown if the correct value would cause overflow.