Fix typo and update robot custom script rule documentation

Change-Id: If81edc1133966f3e55316b7572ebc955364699ef
Signed-off-by: George Keishing <gkeishin@in.ibm.com>
diff --git a/ipmi/test_ipmi_sel.robot b/ipmi/test_ipmi_sel.robot
index 3eaa3de..a9262a3 100644
--- a/ipmi/test_ipmi_sel.robot
+++ b/ipmi/test_ipmi_sel.robot
@@ -29,7 +29,7 @@
     [Documentation]  Verify IPMI sel clear command clears the SEL entry.
     [Tags]  Verify_Empty_SEL
 
-    # Generate an error log and verify there is one atleast.
+    # Generate an error log and verify there is one at least.
     Create Test PEL Log
     ${resp}=  Run IPMI Standard Command  sel elist last 1
     Log To Console  ${resp}