Fix LaTeX equation in corfunc docs

[llimeht]

Description

Fix typo in LaTeX equation. Prevents compilation of documentation with LaTeX in 6.0.0 release; doesn't render properly in mathjax.

How Has This Been Tested?

Build docs using sphinx / pdflatex

Review Checklist:

Documentation (check at least one)

• There is nothing that needs documenting
• Documentation changes are in this PR
• There is an issue open for the documentation (link?)

Installers

• There is a chance this will affect the installers, if so
  • Windows installer (GH artifact) has been tested (installed and worked)
  • MacOSX installer (GH artifact) has been tested (installed and worked)

Licencing (untick if necessary)

• The introduced changes comply with SasView license (BSD 3-Clause)