message: Fix GLSL example in program macro docs
  Looks like someone mistook this for rust code :)
diff: @@ -486,7 +486,7 @@ macro_rules! implement_uniform_block {
  /// vertex: r#"
  /// #version 300
  ///
  -/// fn main() {
  +/// void main() {
  /// gl_Position = vec4(0.0, 0.0, 0.0, 1.0);
  /// }
  /// "#,
  @@ -494,7 +494,7 @@ macro_rules! implement_uniform_block {
  /// #version 300
  ///
  /// out vec4 color;
  -/// fn main() {
  +/// void main() {
  /// color = vec4(1.0, 1.0, 0.0, 1.0);
  /// }
  /// "#,
  @@ -503,14 +503,14 @@ macro_rules! implement_uniform_block {
  /// vertex: r#"
  /// #version 110
  ///
  -/// fn main() {
  +/// void main() {
  /// gl_Position = vec4(0.0, 0.0, 0.0, 1.0);
  /// }
  /// "#,
  /// fragment: r#"
  /// #version 110
  ///
  -/// fn main() {
  +/// void main() {
  /// gl_FragColor = vec4(1.0, 1.0, 0.0, 1.0);
  /// }
  /// "#,
  @@ -519,14 +519,14 @@ macro_rules! implement_uniform_block {
  /// vertex: r#"
  /// #version 110
  ///
  -/// fn main() {
  +/// void main() {
  /// gl_Position = vec4(0.0, 0.0, 0.0, 1.0);
  /// }
  /// "#,
  /// fragment: r#"
  /// #version 110
  ///
  -/// fn main() {
  +/// void main() {
  /// gl_FragColor = vec4(1.0, 1.0, 0.0, 1.0);
  /// }
  /// "#,
  