Docs: Make wording familiar to Salesforce admins

What does this MR do?

Updates for https://docs.gitlab.com/ee/integration/salesforce.html

Hi, I work at Salesforce on our Trailhead team. I've been collaborating with @francispotter and @mayanktahil from GitLab on how to use Salesforce and GitLab for continuous integration.

Thanks for adding this documentation and helping get the Salesforce SSO option available for our mutual customers. I'm suggesting this merge request with some minor verbiage tweaks to make the content a bit more familiar to Salesforce admins.

Please let me know if you have any questions, thanks!

Changes

• Makes wording familiar to Salesforce admins.
• Uses "Salesforce" (lowercase 'f')
• Uses Salesforce help doc language to navigate setup menu to create a connected app.
• Clarifies to use callback url matching the GitLab installation url

Related issues

Author's checklist

• Follow the Documentation Guidelines and Style Guide.
• Link docs to and from the higher-level index page, plus other related docs where helpful.
• Apply the ~Documentation label.

Review checklist

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