sandbox: Fix a comment in os_find_u_boot()

Fix a missing dot in a comment, since '..' is confusing.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>

arch/sandbox/cpu/os.c

@@ -964,7 +964,7 @@ int os_find_u_boot(char *fname, int maxlen, bool use_img,
 	p = strstr(fname, subdir);
 	if (p) {
 		if (*next_prefix)
-			/* e.g. ".../tpl/u-boot-spl"  to "../spl/u-boot-spl" */
+			/* e.g. ".../tpl/u-boot-spl"  to ".../spl/u-boot-spl" */
 			memcpy(p + 1, next_prefix, strlen(next_prefix));
 		else
 			/* e.g. ".../spl/u-boot" to ".../u-boot" */