runtime: tweak doc for Goexit

Use singular form of panic and remove the unnecessary
'however', when comparing Goexit's behavior to 'a panic'
as well as what happens for deferred recovers with Goexit.

Change-Id: I3116df3336fa135198f6a39cf93dbb88a0e2f46e
Reviewed-on: https://go-review.googlesource.com/79755Reviewed-by: Rob Pike <r@golang.org>

src/runtime/panic.go

@@ -347,7 +347,7 @@ func deferreturn(arg0 uintptr) {
 
 // Goexit terminates the goroutine that calls it. No other goroutine is affected.
 // Goexit runs all deferred calls before terminating the goroutine. Because Goexit
-// is not panic, however, any recover calls in those deferred functions will return nil.
+// is not a panic, any recover calls in those deferred functions will return nil.
 //
 // Calling Goexit from the main goroutine terminates that goroutine
 // without func main returning. Since func main has not returned,