Fix Writer exemplar in docs

doc/types/Parser.qbk

@@ -30,7 +30,7 @@ In this table:
     [`a.complete()`]
     [`bool`]
     [
-        Returns `true` when the parser is complete.
+        Returns `true` when parsing is complete.
     ]
 ]
 [

doc/types/Writer.qbk

@@ -128,8 +128,8 @@ public:
         use chunk-encoding or terminate the connection to indicate the end
         of the message.
     */
-    std::size_t
-    content_length() const;
+    std::uint64_t
+    content_length();
 
     /** Write zero or one buffer representing the message body.