doc fix: raises accepts tuples instead of lists

[hoefling]

Fixes #3907.

[coveralls]

Coverage increased (+0.05%) to 92.238% when pulling 8c96eea on hoefling:docfix into 338953a on pytest-dev:master.

[nicoddemus]

Thanks!

[codecov]

Codecov Report

Merging #3910 into master will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3910      +/-   ##
==========================================
+ Coverage   92.18%   92.23%   +0.04%     
==========================================
  Files          52       52              
  Lines       10009    10009              
==========================================
+ Hits         9227     9232       +5     
+ Misses        782      777       -5

Impacted Files	Coverage Δ
src/_pytest/terminal.py	94.23% <0%> (+0.92%)	⬆️

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