docs: Iterator -> Enumerator

There's no such thing as an `Iterator` in Ruby.
What these methods actually return is an `Enumerator`.

Author: alyssais

ext/rugged/rugged_repo.c

@@ -1589,7 +1589,7 @@ static int rugged__each_id_cb(const git_oid *id, void *payload)
 /*
  *  call-seq:
  *    repo.each_id { |id| block }
- *    repo.each_id -> Iterator
+ *    repo.each_id -> Enumerator
  *
  *  Call the given +block+ once with every object ID found in +repo+
  *  and all its alternates. Object IDs are passed as 40-character

ext/rugged/rugged_revwalk.c

@@ -428,13 +428,13 @@ static VALUE rb_git_walk_with_opts(int argc, VALUE *argv, VALUE self, int oid_on
 /*
  *  call-seq:
  *    walker.each { |commit| block }
- *    walker.each -> Iterator
+ *    walker.each -> Enumerator
  *
  *  Perform the walk through the repository, yielding each
  *  one of the commits found as a <tt>Rugged::Commit</tt> instance
  *  to +block+.
  *
- *  If no +block+ is given, an +Iterator+ will be returned.
+ *  If no +block+ is given, an +Enumerator+ will be returned.
  *
  *  The walker must have been previously set-up before a walk can be performed
  *  (i.e. at least one commit must have been pushed).
@@ -458,13 +458,13 @@ static VALUE rb_git_walker_each(int argc, VALUE *argv, VALUE self)
 /*
  *  call-seq:
  *    walker.each_oid { |commit| block }
- *    walker.each_oid -> Iterator
+ *    walker.each_oid -> Enumerator
  *
  *  Perform the walk through the repository, yielding each
  *  one of the commit oids found as a <tt>String</tt>
  *  to +block+.
  *
- *  If no +block+ is given, an +Iterator+ will be returned.
+ *  If no +block+ is given, an +Enumerator+ will be returned.
  *
  *  The walker must have been previously set-up before a walk can be performed
  *  (i.e. at least one commit must have been pushed).

ext/rugged/rugged_tree.c

@@ -260,7 +260,7 @@ static int rugged__treewalk_cb(const char *root, const git_tree_entry *entry, vo
 /*
  *  call-seq:
  *    tree.walk(mode) { |root, entry| block }
- *    tree.walk(mode) -> Iterator
+ *    tree.walk(mode) -> Enumerator
  *
  *  Walk +tree+ with the given mode (either +:preorder+ or +:postorder+) and yield
  *  to +block+ every entry in +tree+ and all its subtrees, as a +Hash+. The +block+
@@ -270,7 +270,7 @@ static int rugged__treewalk_cb(const char *root, const git_tree_entry *entry, vo
  *  If the +block+ returns a falsy value, that entry and its sub-entries (in the case
  *  of a folder) will be skipped for the iteration.
  *
- *  If no +block+ is given, an +Iterator+ is returned instead.
+ *  If no +block+ is given, an +Enumerator+ is returned instead.
  *
  *    tree.walk(:postorder) { |root, entry| puts "#{root}#{entry[:name]} [#{entry[:oid]}]" }
  *