Fix generic window parameter in example

Due to Rust issue [17567](https://github.com/rust-lang/rust/issues/17567) using
a generic parameter like this can lead to very confusing error message here,
when one expands App by methods that do not take a Window implementation as a
parameter.
Merge pull request #14 from aepsil0n/getting-started/method_genericity

Fix generic window parameter in example