Fix typos

[Marcofann]

This pull request addresses grammatical typos in two source files: constraint_system.rs and linear_proof.rs. These changes enhance the clarity and accuracy of the comments in the code.

Changes Made:

1. linear_proof.rs:

   • Corrected "an 'lightweight'" to "a 'lightweight'" in the comment describing a linear proof.

2. constraint_system.rs:

   • Updated "on a even call" to "on an even call" in the comment explaining multiplier assignments.