translation-en.json: add colon to "custom" label
The styling of the text field that appears after this label
makes it easy to miss as an input field.
Having the colon after the label makes it clearer
that the user can type into the text box.
"col-separated-by": "Columns are separated by",
"commas": "commas (CSV)",
"tabs": "tabs (TSV)",
- "custom": "custom",
+ "custom": "custom:",
"escape": "Escape special characters with \\",
"use-quote": "Use character",
"quote-delimits-cells": "to enclose cells containing column separators",
