Fix little typo on docs

[albertoalmagro]

While reading the source file documentation I spotted this little typo.

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[albertoalmagro]

I signed it!

[googlebot]

CLAs look good, thanks!

[coveralls]

Coverage remained the same at 96.081% when pulling 9f16b63 on albertoalmagro:fix-json-representation-typo into b121c39 on google:master.

[bantini]

@sqrrrl Looks like something that should definitely be merged?