Fix "--" in "Securing your webhooks" topic

[docs-bot]

What feature or product is affected?

"Securing your webhooks"

What needs changed?

The first paragraph has: "... about this--for example, you could opt to allow requests from GitHub's IP address--but a far easier ..."

The double hyphens were probably meant to be em dashes, but our style is either to rewrite the sentence to avoid overlong sentences, or to put the subordinate clause in parentheses:

"... about this (for example, you could opt to allow requests from GitHub's IP address) but a far easier ..."

Who does this affect?

All users

What is the impact to users (include Hubbers, such as Support and Sales in this)?

Very minor

Versioning

All