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

This affects documentation and code comments.
diff --git a/Doc/library/xml.etree.elementtree.rst b/Doc/library/xml.etree.elementtree.rst
index 212a9e1..b588598 100644
--- a/Doc/library/xml.etree.elementtree.rst
+++ b/Doc/library/xml.etree.elementtree.rst
@@ -901,7 +901,7 @@
    to get proper namespace handling on output.  *text_or_uri* is a string
    containing the QName value, in the form {uri}local, or, if the tag argument
    is given, the URI part of a QName.  If *tag* is given, the first argument is
-   interpreted as an URI, and this argument is interpreted as a local name.
+   interpreted as a URI, and this argument is interpreted as a local name.
    :class:`QName` instances are opaque.