[ObjC] Fix up clang-format directives in generate files.

- If the source name is long enough, clang-format can want to reflow that
  comment line, so move up the directive.
- Fix typo at the end of the file.

objectivec/GPBDuration.pbobjc.h

diff --git a/objectivec/GPBDuration.pbobjc.h b/objectivec/GPBDuration.pbobjc.h
index fda9b47..7fb2658 100644
--- a/objectivec/GPBDuration.pbobjc.h
+++ b/objectivec/GPBDuration.pbobjc.h
@@ -1,7 +1,6 @@
 // Generated by the protocol buffer compiler.  DO NOT EDIT!
-// source: google/protobuf/duration.proto
-
 // clang-format off
+// source: google/protobuf/duration.proto
 
 #import "GPBDescriptor.h"
 #import "GPBMessage.h"