registrar: fixing typo in comment

I was just going through the module and noticed the typo. This commit
fixes 'onfalure' -> 'onfailure'.

Differential Revision: https://phab.mercurial-scm.org/D906

mercurial/registrar.py

@@ -347,7 +347,7 b' class internalmerge(_funcregistrarbase):'
     fullmerge   o        o        o
     ========== ======== ======== =========
 
-    Optional argument 'onfalure' is the format of warning message
+    Optional argument 'onfailure' is the format of warning message
     to be used at failure of merging (target filename is specified
     at formatting). Or, None or so, if warning message should be
     suppressed.