Say "Bad file descriptor" rather than "Bad file number".

This text is actually in POSIX (though it's not mandatory) and it's what glibc
says. Who says "file number" anyway?

Change-Id: Icc91ac24587c2bc692e0b97c19d32ac1bdda4ea7
diff --git a/libc/include/sys/_errdefs.h b/libc/include/sys/_errdefs.h
index 3eeadf4..3c3c9d7 100644
--- a/libc/include/sys/_errdefs.h
+++ b/libc/include/sys/_errdefs.h
@@ -45,7 +45,7 @@
 __BIONIC_ERRDEF( ENXIO          ,   6, "No such device or address" )
 __BIONIC_ERRDEF( E2BIG          ,   7, "Argument list too long" )
 __BIONIC_ERRDEF( ENOEXEC        ,   8, "Exec format error" )
-__BIONIC_ERRDEF( EBADF          ,   9, "Bad file number" )
+__BIONIC_ERRDEF( EBADF          ,   9, "Bad file descriptor" )
 __BIONIC_ERRDEF( ECHILD         ,  10, "No child processes" )
 __BIONIC_ERRDEF( EAGAIN         ,  11, "Try again" )
 __BIONIC_ERRDEF( ENOMEM         ,  12, "Out of memory" )