Add space to plasma error message

[arjunsavel]

Description

I've added a space to an error message in plasma/standard_plasmas.py.

Motivation and Context

When the link in this error message is printed out, it currently includes ",for" at the end of the link itself — leading the user to a broken page.

How Has This Been Tested?

N/A (I think!)

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 not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have assigned/requested two reviewers for this pull request.

[codecov]

Codecov Report

Merging #1274 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1274   +/-   ##
=======================================
  Coverage   80.57%   80.57%           
=======================================
  Files          41       41           
  Lines        3428     3428           
=======================================
  Hits         2762     2762           
  Misses        666      666           

Impacted Files	Coverage Δ
tardis/plasma/standard_plasmas.py	83.95% <ø> (ø)

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