runtime: fix typo in comments

Fix typo in word synchronization in comments.

Change-Id: I453b4e799301e758799c93df1e32f5244ca2fb84
Reviewed-on: https://go-review.googlesource.com/25174Reviewed-by: Russ Cox <rsc@golang.org>

src/runtime/mgc.go

@@ -235,7 +235,7 @@ func setGCPercent(in int32) (out int32) {
 }
 
 // Garbage collector phase.
-// Indicates to write barrier and sychronization task to preform.
+// Indicates to write barrier and synchronization task to perform.
 var gcphase uint32
 
 // The compiler knows about this variable.