Fix path

Author: ksss

core/encoding.rbs

@@ -923,7 +923,9 @@ class Encoding::Converter < Object
   #     p ec.primitive_convert(src, dst, nil, 1)             #=> :destination_buffer_full
   #     p ec.last_error      #=> nil
   #
-  def last_error: () -> (InvalidByteSequenceError | UndefinedConversionError | nil)
+  def last_error: () -> ( Encoding::InvalidByteSequenceError
+                        | Encoding::UndefinedConversionError
+                        | nil )
 
   # <!--
   #   rdoc-file=transcode.c