Fix documentation about external-dns service

[giovannicandido]

I'm using HostPort type ingress.
For me annotation on service works, not on pod.

Using external-dns version 0.5.2

[codecov-io]

Codecov Report

Merging #1120 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #1120   +/-   ##
======================================
  Coverage    3.64%   3.64%           
======================================
  Files         106     106           
  Lines       21010   21010           
======================================
  Hits          765     765           
  Misses      20114   20114           
  Partials      131     131

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[tamalsaha]

Thanks @giovannicandido !