fix func(Context) error to HandlerFunc

[lkeix]

HandlerFunc has already been defined as func(Context) error in echo.go.
I think using HandlerFunc is more smarter than func(Context) error!

[codecov]

Codecov Report

Base: 92.35% // Head: 92.35% // No change to project coverage 👍

Coverage data is based on head (48b2f2f) compared to base (8f2bf82).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2315   +/-   ##
=======================================
  Coverage   92.35%   92.35%           
=======================================
  Files          37       37           
  Lines        4436     4436           
=======================================
  Hits         4097     4097           
  Misses        247      247           
  Partials       92       92           

Impacted Files	Coverage Δ
echo.go	95.18% <100.00%> (ø)

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