S. 80G: Donation-Approval -Application under wrong sub-clause -Curable defect -Commissioner (Exemptions) must permit rectification -Approval cannot be denied for technical error-Duty of Officers to assist assesses in obtaining relief to which assessee entitled-CBDT Circular No. 14 (XL-35), dated 11-4-1955 [S.12A, 80G(5) S. 119)]