fix sed for macOS

CHANGELOG.md

@@ -146,8 +146,9 @@ Changes in Coq:
 * Changed `frac_auth` notation from `●!`/`◯!` to `●F`/`◯F`. sed script:
   `s/◯!/◯F/g; s/●!/●F/g;`.
 * Rename `C` suffixes into `O` since we no longer use COFEs but OFEs. Also
-  rename `ofe_fun` into `discrete_fun` and the corresponding notation `-c>`
-  into `-d>`. The renaming can be automatically done using the following script:
+  rename `ofe_fun` into `discrete_fun` and the corresponding notation `-c>` into
+  `-d>`. The renaming can be automatically done using the following script (on
+  macOS you will have to replace the `-i` at the end by `-i ''`):
 ```
 sed '
 s/\bCofeMor/OfeMor/g;