docs: add 'ts' tag to code blocks in manual/examples/os_signals.md (#5297)

This should enable syntax highlighting on the website.

docs/examples/os_signals.md

@@ -7,7 +7,7 @@
 
 You can use `Deno.signal()` function for handling OS signals.
 
-```
+```ts
 for await (const _ of Deno.signal(Deno.Signal.SIGINT)) {
   console.log("interrupted!");
 }
@@ -15,7 +15,7 @@ for await (const _ of Deno.signal(Deno.Signal.SIGINT)) {
 
 `Deno.signal()` also works as a promise.
 
-```
+```ts
 await Deno.signal(Deno.Singal.SIGINT);
 console.log("interrupted!");
 ```
@@ -23,9 +23,11 @@ console.log("interrupted!");
 If you want to stop watching the signal, you can use `dispose()` method of the
 signal object.
 
-```
+```ts
 const sig = Deno.signal(Deno.Signal.SIGINT);
-setTimeout(() => { sig.dispose(); }, 5000);
+setTimeout(() => {
+  sig.dispose();
+}, 5000);
 
 for await (const _ of sig) {
   console.log("interrupted");