Fix incorrect use of wildcard in gitignore
Fix incorrect use of wildcard in gitignore

The components/builder-web/.gitignore file was incorrectly using
`app**/*` instead of `app/**/*`.

Running ripgrep on the repo caused the following warning to be printed:

```
error parsing glob 'app**/*.js': invalid use of **; must be one path component
```

Signed-off-by: Lorenzo Manacorda <86995a091f8a1385082bd10715048b52ee386a6a@kinvolk.io>

habitat-sh/habitat,rsertelon/habitat,georgemarshall/habitat,nathenharvey/habitat,georgemarshall/habitat,rsertelon/habitat,nathenharvey/habitat,habitat-sh/habitat,nathenharvey/habitat,habitat-sh/habitat,nathenharvey/habitat,georgemarshall/habitat,rsertelon/habitat,rsertelon/habitat,habitat-sh/habitat,habitat-sh/habitat,habitat-sh/habitat,georgemarshall/habitat,georgemarshall/habitat,nathenharvey/habitat,habitat-sh/habitat,georgemarshall/habitat,rsertelon/habitat,nathenharvey/habitat,georgemarshall/habitat,rsertelon/habitat,georgemarshall/habitat,habitat-sh/habitat,rsertelon/habitat