Fix warnings when testing documentation

Author: khvzak

src/function.rs

@@ -146,7 +146,7 @@ impl Function {
     ///     Ok(())
     /// })?;
     ///
-    /// sleep.call_async(10).await?;
+    /// sleep.call_async::<()>(10).await?;
     ///
     /// # Ok(())
     /// # }

src/state.rs

@@ -639,7 +639,7 @@ impl Lua {
     ///     .into_function()?,
     /// )?;
     /// while co.status() == ThreadStatus::Resumable {
-    ///     co.resume(())?;
+    ///     co.resume::<()>(())?;
     /// }
     /// # Ok(())
     /// # }
@@ -1904,7 +1904,7 @@ impl Lua {
     /// fn main() -> Result<()> {
     ///     let lua = Lua::new();
     ///     lua.set_app_data("hello");
-    ///     lua.create_function(hello)?.call(())?;
+    ///     lua.create_function(hello)?.call::<()>(())?;
     ///     let s = lua.app_data_ref::<&str>().unwrap();
     ///     assert_eq!(*s, "world");
     ///     Ok(())

src/thread.rs

@@ -445,7 +445,7 @@ impl Thread {
     ///     Ok(())
     /// })?)?;
     /// thread.sandbox()?;
-    /// thread.resume(())?;
+    /// thread.resume::<()>(())?;
     ///
     /// // The global environment should be unchanged
     /// assert_eq!(lua.globals().get::<Option<u32>>("var")?, None);