Document the correct loglevel for the heartbeat message

changes/doc-heartbeat-loglevel

@@ -0,0 +1,3 @@
+  o Minor documentation fixes:
+    - Fix the documentation of HeartbeatPeriod to say that the heartbeat
+      message is logged at notice, not at info.

doc/tor.1.txt

@@ -1530,7 +1530,7 @@ is non-zero):
 
 **HeartbeatPeriod**  __N__ **minutes**|**hours**|**days**|**weeks**::
     Log a heartbeat message every **HeartbeatPeriod** seconds. This is
-    a log level __info__ message, designed to let you know your Tor
+    a log level __notice__ message, designed to let you know your Tor
     server is still alive and doing useful things. Settings this
     to 0 will disable the heartbeat. (Default: 6 hours)