ArrayDeque.alloc doc: do not use macro syntax

`$len` is a macro injection while `len` is not a macro defined somewhere

Author: unkarjedy

library/src/scala/collection/mutable/ArrayDeque.scala

@@ -554,7 +554,7 @@ object ArrayDeque extends StrictOptimizedSeqFactory[ArrayDeque] {
  private[ArrayDeque] final val StableSize = 128
 
  /**
- * Allocates an array whose size is next power of 2 > $len
+ * Allocates an array whose size is next power of 2 > `len`
  * Largest possible len is 1<<30 - 1
  *
  * @param len