Remove unneeded variable from example

Apparently doctests swallow *all* of their output, including compiler
warnings, by default.

src/lua.rs

@@ -207,7 +207,6 @@ impl<'lua> String<'lua> {
     /// # use rlua::{Lua, String};
     /// # fn main() {
     /// let lua = Lua::new();
-    /// let globals = lua.globals();
     ///
     /// let non_utf8: String = lua.eval(r#"  "test\xff"  "#, None).unwrap();
     /// assert!(non_utf8.to_str().is_err());    // oh no :(