fiber.c: fix grammar
`suspend` -> `suspended`
* call-seq:
* Fiber.new{...} -> obj
*
- * Creates a fiber, whose execution is suspend until it is explicitly
+ * Creates a fiber, whose execution is suspended until it is explicitly
* resumed using <code>Fiber#resume</code> method.
* The code running inside the fiber can give up control by calling
* <code>Fiber.yield</code> in which case it yields control back to caller
