Typo fix for Install.md

[mapleFU]

Please note that for legal reasons we require you to sign the Contributor Agreement before we can accept your pull requests.

Code changes and contributions in the VelocyPack library should use the "Google" style and conventions as used by clang-format.

[cla-bot[bot]]

Fresh avocado detected! Welcome and thank you for your contribution @mapleFU. My avocado-loving overlords have decreed a signed CLA is required for PRs. Please see https://github.com/arangodb/arangodb/blob/devel/CONTRIBUTING.md file to learn more or ask @cw00dw0rd if you have issues.