api-trace.txt: fix typo

The correct api is trace_printf_key(), not trace_print_key().

Also do not throw a random string at printf(3)-like function;
instead, feed it as a parameter that is fed to a "%s" conversion
specifier.

Signed-off-by: Elia Pinto <gitter.spiros@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/api-trace.txt

@@ -28,7 +28,7 @@ static struct trace_key trace_foo = TRACE_KEY_INIT(FOO);
 
 static void trace_print_foo(const char *message)
 {
-	trace_print_key(&trace_foo, message);
+	trace_printf_key(&trace_foo, "%s", message);
 }
 ------------
 +