plugin/clouddns: tiny fixes for the README

[miekg]

Did a post-merge review. Fix a few typos.

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[codecov-io]

Codecov Report

Merging #3140 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3140      +/-   ##
==========================================
+ Coverage   55.73%   55.76%   +0.02%     
==========================================
  Files         210      210              
  Lines       10516    10516              
==========================================
+ Hits         5861     5864       +3     
+ Misses       4229     4227       -2     
+ Partials      426      425       -1

Impacted Files	Coverage Δ
plugin/route53/route53.go	86.61% <0%> (+2.11%)	⬆️

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