[SERIALIZATION] updated _abstract file (#2306)

Co-authored-by: pf <pf@pf-ubuntu-dev>

dlib/vectorstream/vectorstream_abstract.h

@@ -42,18 +42,24 @@ namespace dlib
                   be used.  Therefore, any time data is written to this stream it will
                   immediately show up in the buffer.
         !*/
-
-        std::istream& seekg (
-            std::streampos pos
+        
+        vectorstream (
+            std::vector<int8_t>& buffer
         );
         /*!
             ensures
-                - The next read from this object will read from the position buffer[pos],
-                  where buffer is the std::vector given to this object's constructor.  Note
-                  that if pos >= buffer.size() then the next read will simply return EOF.
-                - returns *this
+                - This constructor has the same contract as the previous constructor
+                  except the buffer is now std::vector<int8_t>
+        !*/
+        
+        vectorstream (
+            std::vector<uint8_t>& buffer
+        );
+        /*!
+            ensures
+                - This constructor has the same contract as the previous constructor
+                  except the buffer is now std::vector<uint8_t>
         !*/
-
     };
 }