streams: Increase margin in the hint for adding streams.

This prevents this hint from looking wrong in languages where the
string ends up longer.

Fixes #19848.

static/styles/subscriptions.css

@@ -481,6 +481,8 @@
             margin-top: calc(45vh - 75px);
             text-align: center;
             font-size: 1em;
+            margin-left: 2em;
+            margin-right: 2em;
 
             span {
                 color: hsl(0, 0%, 67%);