[DOC] Fix missing word [ci skip]

Co-authored-by: Olle Jonsson <olle.jonsson@gmail.com>

lib/random/formatter.rb

@@ -234,7 +234,7 @@ def alphanumeric(n=nil)
   # The result will consist of chunks upto _chunk_ bytes separated by
   # the elements of _separators_.  The chunks consist of alpha-numeric
   # characters, except for the elements of _exclude_ unless it is
-  # falsy.  The last chunk may longer than _chunk_.  _separators_
+  # falsy.  The last chunk may be longer than _chunk_.  _separators_
   # should be an Array of one-byte Strings, or a String.  The defaults
   # of _chunk_, _separators_ and _exclude_ are CHUNK_SIZE, PUNCT and
   # EXCLUDE respectively.