help: don't crash when nano was started with --noread

There is no need to restore NOREAD_MODE after unsetting it, because
any files mentioned on the command line have already been /not/ read.

src/help.c

@@ -142,6 +142,7 @@ void do_help(void)
     }
 
     UNSET(WHITESPACE_DISPLAY);
+    UNSET(NOREAD_MODE);
 
     bottombars(MHELP);
     wnoutrefresh(bottomwin);