Update include/ruby/thread.h

Co-authored-by: Jean Boussier <jean.boussier@gmail.com>

include/ruby/thread.h

@@ -277,7 +277,7 @@ typedef int rb_internal_thread_specific_key_t;
  * can be created. raises `ThreadError` if exceeded.
  *
  * Usage:
- *   // at initialize timing:
+ *   // at initialize time:
  *   int tool_key; // gvar
  *   Init_tool() {
  *     tool_key = rb_internal_thread_specific_key_create();