Documentation fixes for setup instructions & typos

[schwentker]

Found a few documentation issues that might affect the dev experience:

Setup issue:

• The git clone command uses a placeholder URL that would prevent users from cloning the repository:
  git clone https://github.com/yourusername/box-pinecone-sample.git

Probably should be:
git clone https://github.com/box-community/box-pinecone-sample.git

Minor typos:

• "insructuctions" should be "instructions" (first paragraph)
• "incumbant" should be "incumbent" (Important section)
• "appropriate run" should be "appropriately run" (setup description)

These are small fixes but the git clone issue might block new devs from getting started with the project.