sed.md: use alternative separators (#806)

pages/common/sed.md

@@ -21,3 +21,7 @@
 - Apply multiple find-replace expressions to a file:
 
 `sed -e 's/{{find}}/{{replace}}/' -e 's/{{find}}/{{replace}}/' {{filename}}`
+
+- Replace separator / by any other character not used in your string, e.g., #:
+
+`sed 's#{{find}}#{{replace}}#' {{filename}}`