Escape double dashes in Doxygen input to preserve them

By default double dashes are converted to en-dashes in the output (and triple
ones -- to em-dashes), but this is undesirable when double dashes are used not
as a punctuation mark but in command line options or as C++ decrement
operator, so escape them to avoid such conversion in this case.

interface/wx/hashmap.h

@@ -146,7 +146,7 @@
     ++it ( and it++ ) to move to the next element, *it to access the element pointed to,
     it->first ( it->second ) to access the key ( value ) of the element pointed to.
 
-    Hash maps provide forward only iterators, this means that you can't use --it,
+    Hash maps provide forward only iterators, this means that you can't use \--it,
     it + 3, it1 - it2.