llvmbot monitor: Add HTML DOCTYPE

Just because I was snooping around in the Chrome
console and saw a warning that there was none.

This prevents some non-standad rendering apparently,
though I'm sure we don't use enough things to notice.

Change-Id: Ifc43b72d6cab1104689b31704965896af1ae2964
diff --git a/monitor/bot-status.py b/monitor/bot-status.py
index 773e7c6..1d2e71c 100755
--- a/monitor/bot-status.py
+++ b/monitor/bot-status.py
@@ -170,6 +170,7 @@
   today = "{}\n".format(datetime.today().ctime())
 
   temp.write(dedent("""\
+          <!DOCTYPE html>
           <style>
             /* Combine the border between cells to prevent 1px gaps
                in the row background colour. */