Try to fix RDoc markup for Enumerator

enumerator.c

@@ -73,7 +73,7 @@
  *   puts %w[foo bar baz].map.with_index { |w, i| "#{i}:#{w}" }
  *   # => ["0:foo", "1:bar", "2:baz"]
  *
- *  == External Iteration
+ * == External Iteration
  *
  * An Enumerator can also be used as an external iterator.
  * For example, Enumerator#next returns the next value of the iterator
@@ -93,18 +93,19 @@
  *
  * External iteration differs *significantly* from internal iteration
  * due to using a Fiber:
- *  - The Fiber adds some overhead compared to internal enumeration.
- *  - The stacktrace will only include the stack from the Enumerator, not above.
- *  - Fiber-local variables are *not* inherited inside the Enumerator Fiber,
- *    which instead starts with no Fiber-local variables.
- *  - Fiber storage variables *are* inherited and are designed
- *    to handle Enumerator Fibers. Assigning to a Fiber storage variable
- *    only affects the current Fiber, so if you want to change state
- *    in the caller Fiber of the Enumerator Fiber, you need to use an
- *    extra indirection (e.g., use some object in the Fiber storage
- *    variable and mutate some ivar of it).
+ * - The Fiber adds some overhead compared to internal enumeration.
+ * - The stacktrace will only include the stack from the Enumerator, not above.
+ * - Fiber-local variables are *not* inherited inside the Enumerator Fiber,
+ *   which instead starts with no Fiber-local variables.
+ * - Fiber storage variables *are* inherited and are designed
+ *   to handle Enumerator Fibers. Assigning to a Fiber storage variable
+ *   only affects the current Fiber, so if you want to change state
+ *   in the caller Fiber of the Enumerator Fiber, you need to use an
+ *   extra indirection (e.g., use some object in the Fiber storage
+ *   variable and mutate some ivar of it).
  *
  * Concretely:
+ *
  *   Thread.current[:fiber_local] = 1
  *   Fiber[:storage_var] = 1
  *   e = Enumerator.new do |y|
@@ -120,7 +121,7 @@
  *   e.each { p _1 }
  *   p Fiber[:storage_var] # => 2 (it ran in the same Fiber/"stack" as the current Fiber)
  *
- *  == Convert External Iteration to Internal Iteration
+ * == Convert External Iteration to Internal Iteration
  *
  * You can use an external iterator to implement an internal iterator as follows:
  *