A few typo fixes (#1716)

Author: NotTheDr01ds

book/custom_commands.md

@@ -270,9 +270,10 @@ Because `def` is a parser keyword, the command name must be known at parse time.
 ```nu
 let name = "foo"
 def $name [] { foo }
-:::
 ```
 
+:::
+
 ### Subcommands
 
 You can also define subcommands of commands using a space. For example, if we wanted to add a new subcommand to [`str`](/commands/docs/str.md), we can create it by naming our subcommand starting with "str ". For example:

book/thinking_in_nu.md

@@ -134,8 +134,8 @@ To make sure the first line is _displayed_, use the [`print` command](/commands/
 
 ```nu
 def latest-file [] {
-print "Returning last file"
-ls | sort-by modified | last
+    print "Returning last file"
+    ls | sort-by modified | last
 }
 ```