Consistently escape "@" in wxRichTextCtrl documentation

The first occurrence of it was escaped, but the second one in each line
was not, resulting in warnings about "unknown command @:".

interface/wx/richtext/richtextprint.h

@@ -44,11 +44,11 @@ enum wxRichTextPageLocation {
     You can specify the following keywords in header and footer text, which will
     be substituted for the actual values during printing and preview.
 
-    - @@DATE@: the current date.
-    - @@PAGESCNT@: the total number of pages.
-    - @@PAGENUM@: the current page number.
-    - @@TIME@: the current time.
-    - @@TITLE@: the title of the document, as passed to the wxRichTextPrinting or
+    - @@DATE@@: the current date.
+    - @@PAGESCNT@@: the total number of pages.
+    - @@PAGENUM@@: the current page number.
+    - @@TIME@@: the current time.
+    - @@TITLE@@: the title of the document, as passed to the wxRichTextPrinting or
       wxRichTextLayout constructor.
 
     @library{wxrichtext}