[ BUG ] ":" symbol generates unnecessary hyphen

[MrR0807]

Describe the bug
The online tool doesn't work with headers like this: "# Chapter 1: Introducing to something". Symbol : is replaced with hyphen, but it should be removed altogether as per explanation.

To Reproduce

# Chapter 1: Introducing
# Chapter 2: The Foundations

Generates:

- [Chapter 1: Introducing](#chapter-1--introducing)
- [Chapter 2: The Foundations](#chapter-2--the-foundations)

Expected behavior
No double hyphen between 1-introducing

- [Chapter 1: Introducing](#chapter-1-introducing)
- [Chapter 2: The Foundations](#chapter-2-the-foundations)

Environment (please complete the following information):
Online tool: https://ecotrust-canada.github.io/markdown-toc/