Fix spelling issue in hroller.rst

'explicitly' was spelled the wrong way

Signed-off-by: Benjamin Kerensa <bkerensa@ubuntu.com>
Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>

man/hroller.rst

@@ -62,7 +62,7 @@ Offline nodes should be ignored.
 Filtering by nodegroup should be allowed.
 
 If instances are online the tool should refuse to do offline rolling
-maintenances, unless explicitely requested.
+maintenances, unless explicitly requested.
 
 End-to-end shelltests should be provided.