[scip-kotlin] Use the correct linguist language alias for Kotlin

The GitHub-embedded syntax highlighting for markdown uses `linguist` to
detect which language to use. For Kotlin there's no alias `kt` so syntax
highlighting doesn't work. Use 'kotlin' to correctly pick up the
language of the signature blocks.

c.f. https://github.com/github-linguist/linguist/blob/master/lib/linguist/languages.yml#L3660
https://github.com/github-linguist/linguist/blob/master/lib/linguist/languages.yml#L3660

semanticdb-kotlinc/src/main/kotlin/com/sourcegraph/semanticdb_kotlinc/SemanticdbTextDocumentBuilder.kt

@@ -163,7 +163,7 @@ class SemanticdbTextDocumentBuilder(
                 is DeclarationDescriptorWithSource -> descriptor.findKDocString() ?: ""
                 else -> ""
             }
-        message = "```kt\n$signature\n```${stripKDocAsterisks(kdoc)}"
+        message = "```kotlin\n$signature\n```${stripKDocAsterisks(kdoc)}"
     }
 
     // Returns the kdoc string with all leading and trailing "/*" tokens removed. Naive