Remove leftover fmt.Errorf() after merging shift width check

I merged two PRs successively, and broke builds:

- eaafb3a "Reject cBPF shifts > 31" added a new errors.Errorf() call.
- 71323da "Replace github.com/pkg/errors with stdlib" dropped the
  errors package.

Change the added errors.Errorf() call over to fmt.Errorf() too.

Author: arthurfabre

cbpfc.go

@@ -398,7 +398,7 @@ func validateInstructions(insns []bpf.Instruction) error {
 			switch i.Op {
 			case bpf.ALUOpShiftLeft, bpf.ALUOpShiftRight:
 				if i.Val >= 32 {
-					return errors.Errorf("instruction %d shifts by %d, must be less than 32: %v", pc, i.Val, insn)
+					return fmt.Errorf("instruction %d shifts by %d, must be less than 32: %v", pc, i.Val, insn)
 				}
 			}
 		}