Updated quickbook docs just fix problems exposed by upgrading to quickbook 1.6

doc/quickbook/counting_iterator.qbk

@@ -152,7 +152,7 @@ operations.
 
   counting_iterator();
 
-[*Requires: ] `Incrementable` is Default Constructible.\n
+[*Requires: ] `Incrementable` is Default Constructible.[br]
 [*Effects: ] Default construct the member `m_inc`.
 
 
@@ -174,13 +174,13 @@ operations.
 
   counting_iterator& operator++();
 
-[*Effects: ] `++m_inc`\n
+[*Effects: ] `++m_inc`[br]
 [*Returns: ] `*this`
 
 
   counting_iterator& operator--();
 
-[*Effects: ] `--m_inc`\n
+[*Effects: ] `--m_inc`[br]
 [*Returns: ] `*this`