Add trailing slash to category links in blog posts

[rlees85]

Description

Currently, when rendering blog posts, the category URLs do not include a trailing slash. For example:
http://localhost/categories/banana
http://localhost/categories/apple

This fix simply corrects this:
http://localhost/categories/banana/
http://localhost/categories/apple/

The sidecar category links already include the trailing slash.

Motivation and Context

• Prevents unnecessary redirect with NGinX
• Allows generally-available lambda@edge function to enable CloudFront with private S3 buckets to display index documents on subfolders. See https://aws.amazon.com/blogs/compute/implementing-default-directory-indexes-in-amazon-s3-backed-amazon-cloudfront-origins-using-lambdaedge/
• Gives consistency with the category links rendered on the sidebar

How Has This Been Tested?

Locally, made change to theme, tested URLs render successfully. They do.

Hugo Version:

$ hugo version
Hugo Static Site Generator v0.46 linux/amd64 BuildDate: 2018-08-02T22:40:09+0200

Browser(s):

• Chrome
• Firefox

Screenshots (if appropriate):

Types of changes

• [*] Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation, including theme.toml, accordingly.
• [*] I have read the Contributing Document.

I don't think many of the boxes above are relevant to this change. No documentation required.