Fix typo in unsigned plugin warning (#28709) (#28722)

Typo fix!

(cherry picked from commit 5b1dbe0b0c)

Co-authored-by: Éamon Ryan <eamonryan@users.noreply.github.com>

public/app/features/plugins/PluginSignatureBadge.tsx

@@ -62,7 +62,7 @@ function getSignatureDisplayModel(signature?: PluginSignatureStatus): BadgeProps
       };
     case PluginSignatureStatus.missing:
       return {
-        text: 'Missing signture',
+        text: 'Missing signature',
         icon: 'exclamation-triangle',
         color: 'red',
         tooltip: 'Missing plugin signature',