fix doc typo for library/test.rst

Doc/library/test.rst

@@ -447,7 +447,7 @@ The :mod:`test.support` module defines the following functions:
    Module and package deprecation messages are suppressed during this import
    if *deprecated* is ``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::