Fix missing brace in placeholder for launch file snippet (#3155)

according to https://github.com/microsoft/vscode/issues/90250, we need to escape closing brace.
diff --git a/package.json b/package.json
index 1fab13b..8f7f4b7 100644
--- a/package.json
+++ b/package.json
@@ -368,7 +368,7 @@
               "type": "go",
               "request": "launch",
               "mode": "debug",
-              "program": "^\"${1:\\${file}}\""
+              "program": "^\"${1:\\${file\\}}\""
             }
           },
           {