doc: fix formatting of function-wrap shell alias

Add a missed backtick to the end of a code segment so that it will be
rendered like preceding examples.

I deeply appreciate the thoroughness of this documentation.  I noticed
the formatting discrepancy reading https://git-scm.com/docs/git-config.

Signed-off-by: Kyle E. Mitchell <kyle@kemitchell.com>
Acked-by: Jean-Noël AVILA <avila.jn@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: kemitchell

Documentation/config/alias.adoc

@@ -38,6 +38,6 @@ it will be treated as a shell command.  For example, defining
 ** A convenient way to deal with this is to write your script
    operations in an inline function that is then called with any
    arguments from the command-line.  For example `alias.cmd = "!c() {
-   echo $1 | grep $2 ; }; c" will correctly execute the prior example.
+   echo $1 | grep $2 ; }; c"` will correctly execute the prior example.
 ** Setting `GIT_TRACE=1` can help you debug the command being run for
    your alias.