transpose typo'd characters

This spelling error was caught by lintian.

Change-Id: I749d4c04acaf4fed64d5a5447e88ea763c46e002
diff --git a/doc/source/zuul.rst b/doc/source/zuul.rst
index 6c77477..7c3e384 100644
--- a/doc/source/zuul.rst
+++ b/doc/source/zuul.rst
@@ -635,7 +635,7 @@
   Default: ``linear``.
 
 **window-increase-factor**
-  DependentPipelineManagers only. The value to be added or mulitplied
+  DependentPipelineManagers only. The value to be added or multiplied
   against the previous window value to determine the new window after
   successful change merges.
   Default: ``1``.