fix(update-methods.md):add escaping to the template so helm doesn't consume it

[galamdring]

If the {{ and }} are not escaped, Helm will consume those, and the whole template will be lost.

The original example:

argocd-image-updater.argoproj.io/git-branch: main:image-updater{{range .Images}}-{{.Name}}-{{.NewTag}}{{end}}

will render as image-updater because the whole template is lost. The fix to this is to escape the {{ and }} like so:

argocd-image-updater.argoproj.io/git-branch: main:image-updater{{"{{"}}range .Images{{"}}"}}-{{"{{"}}.Name{{"}}"}}-{{"{{"}}.NewTag{{"}}"}}{{"{{"}}end{{"}}"}}

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.95%. Comparing base (02eee1d) to head (f1976ce).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #888   +/-   ##
=======================================
  Coverage   75.95%   75.95%           
=======================================
  Files          31       31           
  Lines        3198     3198           
=======================================
  Hits         2429     2429           
  Misses        635      635           
  Partials      134      134           

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[chengfang]

Would it make sense to keep the original form for non-helm cases, and also for easy viewing of the real value? We can add the helm-escaped form for helm cases.