Correct max post size on slash command page

[robinsdm]

Correct the maximum post size documented on the slash commands page. The v5.0 release notes say it is 16383 characters.

[mattermost-build]

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[robinsdm]

/check-cla

[github-actions[bot]]

Newest code from mattermost-build has been published to preview environment for Git SHA e7a8aa85f95d18d97e8218ab651ccf247197be0e

[github-actions[bot]]

Newest code from cwarnermm has been published to preview environment for Git SHA e7a8aa85f95d18d97e8218ab651ccf247197be0e

[github-actions[bot]]

Newest code from cwarnermm has been published to preview environment for Git SHA e7a8aa85f95d18d97e8218ab651ccf247197be0e