libfdt: fix description of fdt_get_string()

Looks like this comment was copied from that of
fdt_get_string_index().

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Fixes: 5094eb408a5d ("fdt: Add functions to retrieve strings")
Acked-by: Simon Glass <sjg@chromium.org>
diff --git a/include/libfdt.h b/include/libfdt.h
index f3cbb63..421d64f 100644
--- a/include/libfdt.h
+++ b/include/libfdt.h
@@ -915,7 +915,7 @@
 			 int index, const char **output);
 
 /**
- * fdt_get_string() - obtain the string at a given index in a string list
+ * fdt_get_string() - obtain the first string in a string list
  * @fdt: pointer to the device tree blob
  * @node: offset of the node
  * @property: name of the property containing the string list