Core: fixed comment about msie_refresh escaping.

After 12a656452ad1, the "%" character is no longer escaped by
ngx_escape_uri(NGX_ESCAPE_REFRESH).

src/core/ngx_string.c

diff --git a/src/core/ngx_string.c b/src/core/ngx_string.c
index 93f32ea..5cc9b26 100644
--- a/src/core/ngx_string.c
+++ b/src/core/ngx_string.c
@@ -1573,7 +1573,7 @@
         0xffffffff  /* 1111 1111 1111 1111  1111 1111 1111 1111 */
     };
 
-                    /* " ", """, "%", "'", %00-%1F, %7F-%FF */
+                    /* " ", """, "'", %00-%1F, %7F-%FF */
 
     static uint32_t   refresh[] = {
         0xffffffff, /* 1111 1111 1111 1111  1111 1111 1111 1111 */