Replace http://man7.org links with https://man7.org links.

I suspect the way http:// dies is that browsers just interpret it as
https:// anyway, but there's no reason not to just bulk fix all these at
source.

Change-Id: I38fa91728846d7c33207ef1bdadd4910eac9e941
diff --git a/libc/include/sys/signalfd.h b/libc/include/sys/signalfd.h
index 2be9bdc..5568b7d 100644
--- a/libc/include/sys/signalfd.h
+++ b/libc/include/sys/signalfd.h
@@ -41,7 +41,7 @@
 __BEGIN_DECLS
 
 /**
- * [signalfd(2)](http://man7.org/linux/man-pages/man2/signalfd.2.html) creates/manipulates a
+ * [signalfd(2)](https://man7.org/linux/man-pages/man2/signalfd.2.html) creates/manipulates a
  * file descriptor for reading signal events.
  *
  * Returns the file descriptor on success, and returns -1 and sets `errno` on failure.