doc: fix timer trigger spelling

Change-Id: I1d788c15d2a64e96ecd82435dcdaef211c3c9d97

doc/source/reference/drivers/timer.rst

@@ -7,8 +7,8 @@ The timer driver supports triggers only.  It is used for configuring
 pipelines so that jobs run at scheduled times.  No connection
 configuration is required.
 
-Trgger Configuration
---------------------
+Trigger Configuration
+---------------------
 
 Timers don't require a special connection or driver. Instead they can
 simply be used by listing ``timer`` as the trigger.