fix doxygen for XLALVectorComputeGLtLDenominator

Description

@karl-wette minor fix for an "Undefined control sequence" and providing a proper sentence for the function along the way.

The whole file is kinda messy LaTeX-wise with some commands being redefined per function and others being reused, but a first brute-force cleanup attempt failed, and I don't want to spend much time on making it nicer for now. So just this isolated fix for now.

API Changes and Justification

Backwards Compatible Changes

• This change does not modify any class/function/struct/type definitions in a public C header file or any Python class/function definitions
• This change adds new classes/functions/structs/types to a public C header file or Python module

Backwards Incompatible Changes

• This change modifies an existing class/function/struct/type definition in a public C header file or Python module
• This change removes an existing class/function/struct/type from a public C header file or Python module

If any of the Backwards Incompatible check boxes are ticked please provide a justification why this change is necessary and why it needs to be done in a backwards incompatible way.

Review Status

n/a