Issue #25523: Correct "a" article to "an" article

This changes the main documentation, doc strings, source code comments, and a
couple error messages in the test suite. In some cases the word was removed
to fix the grammar.
diff --git a/Doc/library/ftplib.rst b/Doc/library/ftplib.rst
index 3ab7b3b..b155daf 100644
--- a/Doc/library/ftplib.rst
+++ b/Doc/library/ftplib.rst
@@ -283,9 +283,9 @@
 
 .. method:: FTP.transfercmd(cmd[, rest])
 
-   Initiate a transfer over the data connection.  If the transfer is active, send a
+   Initiate a transfer over the data connection.  If the transfer is active, send an
    ``EPRT`` or  ``PORT`` command and the transfer command specified by *cmd*, and
-   accept the connection.  If the server is passive, send a ``EPSV`` or ``PASV``
+   accept the connection.  If the server is passive, send an ``EPSV`` or ``PASV``
    command, connect to it, and start the transfer command.  Either way, return the
    socket for the connection.