Fix typos in comments for clarity and grammatical correctness

[defitricks]

Changes:

1. In frontend/builder.go:

   • Fixed the grammatical error in the comment for the MarkBoolean method. The original comment was:

     "MarkBoolean sets (but do not constraint!) v to be boolean"

   • Corrected to:

     "MarkBoolean sets (but does not constrain!) v to be boolean"

   • Explanation: The word "constraint" was used incorrectly. The verb should be "constrain" as it follows the auxiliary verb "does" which requires the infinitive form.

2. In frontend/variable.go:

   • Fixed the plural form of "builder" in the comment for the IsCanonical function. The original comment was:

     "by one of the constraint system builder."

   • Corrected to:

     "by one of the constraint system builders."

   • Explanation: The sentence refers to multiple potential builders, so the correct form is "builders" (plural).

These changes improve the grammatical accuracy and clarity of the comments, ensuring the documentation aligns with standard English usage.

• Bug fix (non-breaking change which fixes an issue)

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[ivokub]

Thanks for the contribution! I'll merge it to the fix/typos-v0.12 branch for now which will be merged before the next release (either 0.11.1 or 0.12). There are some other PRs open and we'd like to avoid individual merge conflicts.