Fix incorrect offset with multi line comment suggestions

app/assets/javascripts/vue_shared/components/markdown/field.vue

@@ -260,7 +260,7 @@ export default {
       :line-content="lineContent"
       :can-suggest="canSuggest"
       :show-suggest-popover="showSuggestPopover"
-      :suggestion-start-index="lines.length"
+      :suggestion-start-index="lines.length - 1"
       @preview-markdown="showPreviewTab"
       @write-markdown="showWriteTab"
       @handleSuggestDismissed="() => $emit('handleSuggestDismissed')"

spec/frontend/snippets/components/__snapshots__/snippet_description_edit_spec.js.snap

@@ -29,7 +29,7 @@ exports[`Snippet Description Edit component rendering matches the snapshot 1`] =
     >
       <markdown-header-stub
         linecontent=""
-        suggestionstartindex="0"
+        suggestionstartindex="-1"
       />
        
       <div