runtime: fix typo in comment

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/140590043

src/runtime/mgc0.c

@@ -997,7 +997,7 @@ runtime·MSpan_Sweep(MSpan *s, bool preserve)
 }
 
 // State of background runtime·sweep.
-// Pretected by runtime·gclock.
+// Protected by runtime·gclock.
 // Must match mgc0.go.
 struct
 {