Fix typo: "an Unicode string" --> "a Unicode string"

Clarify the return value when the parameter is a Unicode object.

Doc/lib/libfuncs.tex

@@ -871,8 +871,8 @@ def my_import(name):
 
   If no optional parameters are given, \code{unicode()} will mimic the
   behaviour of \code{str()} except that it returns Unicode strings
-  instead of 8-bit strings. More precisely, if \var{object} is an
-  Unicode string or subclass it will return a Unicode string without
+  instead of 8-bit strings. More precisely, if \var{object} is a
+  Unicode string or subclass it will return that Unicode string without
   any additional decoding applied.
 
   For objects which provide a \method{__unicode__()} method, it will