Documentation/config.txt: fix typo in core.alternateRefsCommand

In [1] Git learned about 'core.alternateRefsCommand', and with it, the
accompanying documentation. However, this documentation included a typo
involving the verb tense of "produced".

Match the tense of the surrounding bits by correcting this typo.

[1]: 89284c1d6c (transport.c: introduce core.alternateRefsCommand,
     2018-10-08)

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -620,7 +620,7 @@ core.alternateRefsCommand::
 	When advertising tips of available history from an alternate, use the shell to
 	execute the specified command instead of linkgit:git-for-each-ref[1]. The
 	first argument is the absolute path of the alternate. Output must contain one
-	hex object id per line (i.e., the same as produce by `git for-each-ref
+	hex object id per line (i.e., the same as produced by `git for-each-ref
 	--format='%(objectname)'`).
 +
 Note that you cannot generally put `git for-each-ref` directly into the config