runtime: fix bug in Caller documentation

R=r
CC=golang-dev
https://golang.org/cl/207110

src/pkg/runtime/extern.go

@@ -24,7 +24,7 @@ func Breakpoint()
 
 // Caller reports file and line number information about function invocations on
 // the calling goroutine's stack.  The argument is the number of stack frames to
-// ascend, with 1 identifying the the caller of Caller.  The return values report the
+// ascend, with 0 identifying the 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(n int) (pc uintptr, file string, line int, ok bool)