Fix typo in test.rst

Reported by Mike Scalora on docs@p.o.
diff --git a/Doc/library/test.rst b/Doc/library/test.rst
index 434ef5a..6872d5f 100644
--- a/Doc/library/test.rst
+++ b/Doc/library/test.rst
@@ -418,7 +418,7 @@
    Module and package deprecation messages are suppressed during this import
    if *deprecated* is :const:`True`.
 
-   This function will raise :exc:`unittest.SkipTest` is the named module
+   This function will raise :exc:`unittest.SkipTest` if the named module
    cannot be imported.
 
    Example use::