t4150: fix warning printed by awk due to escaped '\@'

In 6aec8d38fd (t: refactor tests depending on Perl to print data,
2025-04-03) we have changed one of the tests in t4150 to use awk(1)
instead of Perl. The test works, but at least gawk(1) prints a warning
now:

    awk: cmd. line:3: warning: escape sequence `\@' treated as plain `@'

Fix this by removing the backslash.

Reported-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4150-am.sh

@@ -1086,7 +1086,7 @@ test_expect_success 'am works with multi-line in-body headers' '
 	# bump from, date, and subject down to in-body header
 	awk "
 		/^From:/{
-			print \"From: x <x\@example.com>\";
+			print \"From: x <x@example.com>\";
 			print \"Date: Sat, 1 Jan 2000 00:00:00 +0000\";
 			print \"Subject: x\n\";
 		}; 1