[fs.filesystem.syn] Update synopsis for filesystem::absolute

Author: jwakely

source/iostreams.tex

@@ -10257,7 +10257,8 @@
   using file_time_type = chrono::time_point<@\textit{trivial-clock}@>;
 
   // \ref{fs.op.funcs}, filesystem operations
-  path absolute(const path& p, const path& base = current_path());
+  path absolute(const path& p);
+  path absolute(const path& p, error_code& ec);
 
   path canonical(const path& p, const path& base = current_path());
   path canonical(const path& p, error_code& ec);