Update variables.mdx

[yaarAdarsh]

docs: Fix spelling mistake in concepts

terraform / opentofu => Terraform/OpenTofu
Plain Text variable => Plain Text variables
Create Secrets Key => Create Secret Key
At no point the private key is saved => At no point is the private key saved
lose ability to decrypt your secrets => lose the ability to decrypt your secrets

Documentation Correction:

• [digger/docs/team/features/variables.mdx]

Corrected typos in the sentence directing users about
the variable reference.

[delve-auditor]

✅ No security or compliance issues detected. Reviewed everything up to 6ba90f2.

Security Overview

• 🔎 Scanned files: 1 changed file(s)

Detected Code Changes

Change Type	Relevant files
Enhancement	► variables.mdx     Update content for Variables & Secrets documentation     Improve clarity and correct minor typos

Reply to this PR with @delve-auditor followed by a description of what change you want and we'll auto-submit a change to this PR to implement it.