Fix typos and markdown (#2388)

* Fix typos

* Align properly

* Update core/consensus/slots/src/lib.rs

Co-Authored-By: cmichi <mich@elmueller.net>

* Update core/network/src/test/mod.rs

Co-Authored-By: cmichi <mich@elmueller.net>

* Update core/finality-grandpa/src/communication/mod.rs

Co-Authored-By: cmichi <mich@elmueller.net>

* Update core/consensus/common/src/import_queue.rs

Co-Authored-By: cmichi <mich@elmueller.net>

substrate/core/util/fork-tree/src/lib.rs

@@ -22,7 +22,7 @@
 use std::fmt;
 use parity_codec::{Decode, Encode};
 
-/// Error occured when interating with the tree.
+/// Error occured when iterating with the tree.
 #[derive(Clone, Debug, PartialEq)]
 pub enum Error<E> {
 	/// Adding duplicate node to tree.