rtl: typo in doc comment

Change-Id: I068d0bce3e0122d4539d489097ef7771f910ed0e
Reviewed-on: https://gerrit.libreoffice.org/40719Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Ashod Nakashian <ashnakash@gmail.com>

include/rtl/ustring.hxx

@@ -443,7 +443,7 @@ public:
     /**
       Move assign a new string.
 
-      @param    str         a OString.
+      @param    str         an OUString.
       @since LibreOffice 5.2
     */
     OUString & operator=( OUString && str )