Mark Out_channel issue as fixed

Author: jmid

README.md

@@ -390,8 +390,8 @@ property can be done in two different ways:
 Issues
 ======
 
-Regression on output to closed `Out_channel`s (new, runtime)
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+Regression on output to closed `Out_channel`s (new, fixed, runtime)
+-------------------------------------------------------------------
 
 While revising out `Out_channel` tests we discovered [a regression when
 outputting to a closed `Out_channel`](https://github.com/ocaml/ocaml/issues/12898)