refactor(shellcheck): fix violation

```
In bin/convert-formula.sh line 90:
        s/========/$(repeat_char =)/
                                 ^-- SC2283 (error): Remove spaces around = to assign
                                                     (or use [ ] to compare,
                                                      or quote '=' if literal).
For more information:
  https://www.shellcheck.net/wiki/SC2283 -- Remove spaces around = to assign ...
```

bin/convert-formula.sh

@@ -87,7 +87,7 @@ convert_formula() {
   # on the following line matches the length of the new formula name
   sedi "/TEMPLATE/{
         n
-        s/========/$(repeat_char =)/
+        s/========/$(repeat_char '=')/
         s/--------/$(repeat_char -)/
         s/\^^^^^^^^/$(repeat_char ^)/
         s/\~~~~~~~~/$(repeat_char \~)/"'