Fix parentheses incorrectly removed around assertion when hanging

[JohnnyMorganz]

Fixes #425

[codecov]

Codecov Report

Merging #426 (906eb2d) into master (80483fe) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #426   +/-   ##
=======================================
  Coverage   96.34%   96.35%           
=======================================
  Files          14       14           
  Lines        5174     5180    +6     
=======================================
+ Hits         4985     4991    +6     
  Misses        189      189           

Impacted Files	Coverage Δ
src/formatters/expression.rs	96.90% <100.00%> (+0.02%)	⬆️

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