Fix textureGatherOffsets parameter type and MathML typo

[Determinated738]

Fix errors in the textureGatherOffsets documentation to bring it into alignment with the official GLSL Specification.

1. gsampler2DRect coordinate parameter should be vec2 instead of vec3. 2D Rectangle samplers use 2D coordinates.
2. In the description, the MathML section should be i0j0 instead of i0i0.

With these changes, it is now consistent with the GLSL Specification https://registry.khronos.org/OpenGL/specs/gl/GLSLangSpec.4.40.pdf#page=178