Update slack invite link

Author: Relequestual

public/_redirects

@@ -21,8 +21,8 @@
 /latest/release-notes /draft/2020-12/release-notes
 /implementations /tools 301
 /obsolete-implementations /tools?showObsolete=true 301
-/slack https://join.slack.com/t/json-schema/shared_invite/zt-2ued3v79g-Tk_aI32ZdW~ST0EWpGBwNQ 301
-/slack-redirect https://join.slack.com/t/json-schema/shared_invite/zt-2ued3v79g-Tk_aI32ZdW~ST0EWpGBwNQ 301
+/slack https://join.slack.com/t/json-schema/shared_invite/zt-3ktcs18gk-ElYFaGDSOv68OJMf3H4vXg 301
+/slack-redirect https://join.slack.com/t/json-schema/shared_invite/zt-3ktcs18gk-ElYFaGDSOv68OJMf3H4vXg 301
 /understanding-json-schema /understanding-json-schema/reference 301
 /understanding-json-schema/conventions /understanding-json-schema/reference 301
-/understanding-json-schema/reference/_index.md /understanding-json-schema/reference 301
+/understanding-json-schema/reference/_index.md /understanding-json-schema/reference 301