log: rename Exit* to Fatal*
This aligns the naming scheme with the testing package and
also lets govet work on more logging calls.

R=rsc
CC=golang-dev
https://golang.org/cl/4001048
diff --git a/doc/htmlgen.go b/doc/htmlgen.go
index 4bed9ed..5d0bad8 100644
--- a/doc/htmlgen.go
+++ b/doc/htmlgen.go
@@ -50,7 +50,7 @@
 			break
 		}
 		if err != nil {
-			log.Exit(err)
+			log.Fatal(err)
 		}
 		n := len(lines)
 		lines = lines[0 : n+1]