Correct spelling of "overridden" in exception messages

[wodow]

[codecov[bot]]

Codecov Report

Merging #9 (d98980d) into master (77f64b0) will increase coverage by 0.65%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master       #9      +/-   ##
==========================================
+ Coverage   90.90%   91.56%   +0.65%     
==========================================
  Files           3        4       +1     
  Lines          77       83       +6     
==========================================
+ Hits           70       76       +6     
  Misses          7        7              

Impacted Files	Coverage Δ
firebase_auth/authentication.py	100.00% <100.00%> (ø)
firebase_auth/__init__.py	100.00% <0.00%> (ø)

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[walison17]

thanks @wodow!