runtime: fix minor typo

Change-Id: I79b7ed8f7e78e9d35b5e30ef70b98db64bc68a7b
Reviewed-on: https://go-review.googlesource.com/7720Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/error.go

@@ -11,9 +11,9 @@ type Error interface {
 	error
 
 	// RuntimeError is a no-op function but
-	// serves to distinguish types that are runtime
+	// serves to distinguish types that are run time
 	// errors from ordinary errors: a type is a
-	// runtime error if it has a RuntimeError method.
+	// run time error if it has a RuntimeError method.
 	RuntimeError()
 }