Update slack invitation link

Author: meteorcloudy

slack-bazel-build/docker/nginx.conf

@@ -18,7 +18,7 @@ http {
         listen      ${PORT};
         server_name slack.bazel.build;
 
-        return 301 https://join.slack.com/t/bazelbuild/shared_invite/zt-2o2h111tz-_PSnG_~Bv7q_yxykQ_HrnA;
+        return 301 https://join.slack.com/t/bazelbuild/shared_invite/zt-2uawevx0j-xM4npo6Fy4qbENP_IDwfdw;
 
         error_page   500 502 503 504  /50x.html;
         location = /50x.html {