docs: add missing backtick in subprocess docs
[stdout](https://doc.deno.land/builtin/stable#Deno.stdout) and
[stderr](https://doc.deno.land/builtin/stable#Deno.stderr) streams.
- Use a specific shell by providing its path/name and its string input switch,
- e.g. `Deno.run({cmd: ["bash", "-c", '"ls -la"']});
+ e.g. `Deno.run({cmd: ["bash", "-c", '"ls -la"']});`
## Simple example
