Update descriptions for BFGS and SR1 operators

[MaxenceGollier]

@tmigot

Updates to tutorial

• Tutorial name: introduction-to-linear-operators
• Confirm that either:
  • There is an issue for this tutorial:
    • #ISSUE_NUMBER
  • It was discussed in some other manner: see Update descriptions for BFGS and SR1 operators JuliaSmoothOptimizers.github.io#216
• Confirm that you have used a tool such as Grammarly to verify that the text is correct.
• Wait for the workflow, look at the preview, and confirm:
  • The preview has your tutorial listed.
  • The preview looks like what you want.