Correct “an” → “a” with “Unicode”, “user”, “UTF”, etc

This affects documentation and code comments.
diff --git a/Doc/library/xml.sax.reader.rst b/Doc/library/xml.sax.reader.rst
index 6956cd1..7270153 100644
--- a/Doc/library/xml.sax.reader.rst
+++ b/Doc/library/xml.sax.reader.rst
@@ -101,7 +101,7 @@
 
    Process an input source, producing SAX events. The *source* object can be a
    system identifier (a string identifying the input source -- typically a file
-   name or an URL), a file-like object, or an :class:`InputSource` object. When
+   name or a URL), a file-like object, or an :class:`InputSource` object. When
    :meth:`parse` returns, the input is completely processed, and the parser object
    can be discarded or reset. As a limitation, the current implementation only
    accepts byte streams; processing of character streams is for further study.