[Editor] Fix variable name in the ftl file

and add a linting rule to avoid such an error in the future.

.github/fluent_linter_config.yml

@@ -25,3 +25,5 @@ CO01:
     exclusions:
         files: []
         messages: []
+VC:
+    disabled: false

l10n/en-US/viewer.ftl

@@ -373,7 +373,7 @@ pdfjs-editor-signature-add-signature-button-label = Add new signature
 
 # Used on the button to use an already saved signature.
 # Variables:
-#   description (String) - a string describing/labeling the signature.
+#   $description (String) - a string describing/labeling the signature.
 pdfjs-editor-add-saved-signature-button =
     .title = Saved signature: { $description }