[9538] Fix eventAsText epytext

[twm]

epydoc didn't like the newlines in the middle of the C{u"{timeStamp} [{system}] {event}\n{traceback}\n"} literal. Making the docstring a raw string changes them to the backslash escapes that were intended anyway.

Contributor Checklist:

• There is an associated ticket in Trac. #9538 (eventAsText docstring is not valid epytext) – Twisted
• The changes pass minimal style checks (see: https://twistedmatrix.com/trac/wiki/TwistedDevelopment#GettingYourPatchAccepted )
• I have created a newsfragment in src/twisted/newsfragments/ (see: https://twistedmatrix.com/trac/wiki/ReviewProcess#Newsfiles )
• I have updated the automated tests. (N/A)

[codecov]

Codecov Report

Merging #1069 into trunk will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##            trunk    #1069      +/-   ##
==========================================
+ Coverage   91.92%   91.92%   +<.01%     
==========================================
  Files         844      844              
  Lines      150865   150865              
  Branches    13157    13157              
==========================================
+ Hits       138680   138684       +4     
+ Misses      10099    10096       -3     
+ Partials     2086     2085       -1

[altendky]

Please merge.

[twm]

Thank you for the review, @altendky!