Fix references to license filename (#6759)

Author: philrz

.goreleaser.yaml

@@ -34,7 +34,7 @@ archives:
       - goos: windows
         formats: [ "zip" ]
     files:
-      - LICENSE.txt
+      - LICENSE.md
       - acknowledgments.txt
 release:
   header: |

sio/jsonio/decode.go

@@ -2,7 +2,7 @@
 // https://github.com/golang/go/blob/master/src/encoding/json/decode.go
 // and is covered by the copyright reproduced below.
 //
-// Any changes are covered by the copyright and license in the LICENSE.txt file
+// Any changes are covered by the copyright and license in the LICENSE.md file
 // in the root directory of this repository.
 
 // Copyright 2010 The Go Authors. All rights reserved.