internal/encoding/messageset: fix spelling typo

Change-Id: Ic397fba6b382fbbe72779aa919f9109c23c8f209
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/259898
Reviewed-by: Damien Neil <dneil@google.com>
Trust: Joe Tsai <thebrokentoaster@gmail.com>
diff --git a/internal/encoding/messageset/messageset.go b/internal/encoding/messageset/messageset.go
index 453a81a..c1866f3 100644
--- a/internal/encoding/messageset/messageset.go
+++ b/internal/encoding/messageset/messageset.go
@@ -13,7 +13,7 @@
 	pref "google.golang.org/protobuf/reflect/protoreflect"
 )
 
-// The MessageSet wire format is equivalent to a message defiend as follows,
+// The MessageSet wire format is equivalent to a message defined as follows,
 // where each Item defines an extension field with a field number of 'type_id'
 // and content of 'message'. MessageSet extensions must be non-repeated message
 // fields.