Fix docs: Set correct go import path for echoprometheus package

[andygrunwald]

The README of the new prometheus middleware states the wrong import path.

[codecov]

Codecov Report

Patch coverage: 77.16% and project coverage change: +4.93 🎉

Comparison is base (b6855c2) 58.84% compared to head (22688b8) 63.77%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #96      +/-   ##
==========================================
+ Coverage   58.84%   63.77%   +4.93%     
==========================================
  Files           8        9       +1     
  Lines         690      944     +254     
==========================================
+ Hits          406      602     +196     
- Misses        260      302      +42     
- Partials       24       40      +16     

Impacted Files	Coverage Δ
prometheus/prometheus.go	49.15% <ø> (ø)
echoprometheus/prometheus.go	77.16% <77.16%> (ø)

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[aldas]

Thanks