Replaces the word fungible with an easier to read synonym like exchangeable on expose-intro.html

[ernestohegi]

While I was reading Using a Service to expose your app I stumbled upon the word "fungible". Its meaning wasn't clear enough for me and stopped my learning process. I'm not sure whether it's a common word but it seems it can be a synonym of "exchangeable", a word I feel is easier to comprehend.

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[ernestohegi]

/assign @steveperry-53

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[jimangel]

/lgtm

[tengqm]

/lgtm
/approve

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