runtime: remove extra parentheses.

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/6843069
diff --git a/src/pkg/runtime/debug.go b/src/pkg/runtime/debug.go
index e9d7601..0211ce6 100644
--- a/src/pkg/runtime/debug.go
+++ b/src/pkg/runtime/debug.go
@@ -4,7 +4,7 @@
 
 package runtime
 
-// Breakpoint() executes a breakpoint trap.
+// Breakpoint executes a breakpoint trap.
 func Breakpoint()
 
 // LockOSThread wires the calling goroutine to its current operating system thread.