swarm-ui: stop suggesting the reserved default account as an example name

The link-matrix-account form's placeholder read 'e.g. primary', which
sounds like it names the hive-provided default account. That account is
actually named main (reserved in hyperhive.matrixAccounts's schema) and
this route can neither create nor touch it. Swapped to an example with
no relationship to the real default, per #4122.

frontend/packages/swarm-ui/src/pages/LinkMatrixAccountForm.tsx

@@ -108,7 +108,13 @@ export function LinkMatrixAccountForm({
           value={account}
           pattern={ACCOUNT_PATTERN}
           title="1-63 chars: letters, digits, underscore, hyphen"
-          placeholder="e.g. primary"
+          // Not "primary" — that reads as naming the hive-provided default
+          // account (`main`, reserved: see `hyperhive.matrixAccounts`'s own
+          // module doc), which this route can neither create nor touch.
+          // Picking a name with no relationship to that default avoids the
+          // misread entirely, rather than requiring a reader to already know
+          // the reservation to see through it.
+          placeholder="e.g. ops-relay"
           required
           onInput={setAccount}
         />