Fix broken link syntax in g_vasprintf docs

There's a broken link in the documentation for the g_vasprintf() function. This fixes it. (It's just a missing bracket)

See what it looks like now: https://developer.gnome.org/glib/2.56/glib-String-Utility-Functions.html#g-vasprintf. Compare this with other functions on the page that also reference the "string precision pitfalls" section.