runtime: fix typo in comment

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/5511047

src/pkg/runtime/extern.go

@@ -19,8 +19,8 @@ func Gosched()
 func Goexit()
 
 // Caller reports file and line number information about function invocations on
-// the calling goroutine's stack.  The argument skip is the number of stack frames to
-// ascend, with 0 identifying the the caller of Caller.  The return values report the
+// the calling goroutine's stack.  The argument skip is the number of stack frames
+// to ascend, with 0 identifying the caller of Caller.  The return values report the
 // program counter, file name, and line number within the file of the corresponding
 // call.  The boolean ok is false if it was not possible to recover the information.
 func Caller(skip int) (pc uintptr, file string, line int, ok bool)