Fix obsolete `completion.snippets` config
//
// rust-analyzer allows the user to define custom (postfix)-snippets that may depend on items to be accessible for the current scope to be applicable.
//
-// A custom snippet can be defined by adding it to the `rust-analyzer.completion.snippets` object respectively.
+// A custom snippet can be defined by adding it to the `rust-analyzer.completion.snippets.custom` object respectively.
//
// [source,json]
// ----
// {
-// "rust-analyzer.completion.snippets": {
+// "rust-analyzer.completion.snippets.custom": {
// "thread spawn": {
// "prefix": ["spawn", "tspawn"],
// "body": [
