Use UTF-8 code for check mark

[adriangonz]

What this PR does / why we need it:

Sphinx doesn't support "emoji codes" like Markdown does. Thus, they get rendered in the docs as:

This PR changes that :heavy_check_mark: into the equivalent UTF-8 code (i.e. ✅).

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[seldondev]

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[seldondev]

Wed Nov 11 16:30:01 UTC 2020
The logs for [pr-build] [1] will show after the pipeline context has finished.
https://github.com/SeldonIO/seldon-core/blob/gh-pages/jenkins-x/logs/SeldonIO/seldon-core/PR-2627/1.log

impatient try
jx get build logs SeldonIO/seldon-core/PR-2627 --build=1

[seldondev]

Wed Nov 11 16:30:03 UTC 2020
The logs for [lint] [2] will show after the pipeline context has finished.
https://github.com/SeldonIO/seldon-core/blob/gh-pages/jenkins-x/logs/SeldonIO/seldon-core/PR-2627/2.log

impatient try
jx get build logs SeldonIO/seldon-core/PR-2627 --build=2