Correct typos that ispell did not find.

doc/crypto/BN_set_bit.pod

@@ -28,12 +28,12 @@ BN_set_bit() sets bit B<n> in B<a> to 1 (C<a|=(1E<lt>E<lt>n)>). The
 number is expanded if necessary.
 
 BN_clear_bit() sets bit B<n> in B<a> to 0 (C<a&=~(1E<lt>E<lt>n)>). An
-error occurs it B<a> is shorter than B<n> bits.
+error occurs if B<a> is shorter than B<n> bits.
 
 BN_is_bit_set() tests if bit B<n> in B<a> is set.
 
 BN_mask_bits() truncates B<a> to an B<n> bit number
-(C<a&=~((~0)E<gt>E<gt>n)>).  An error occurs it B<a> already is
+(C<a&=~((~0)E<gt>E<gt>n)>).  An error occurs if B<a> already is
 shorter than B<n> bits.
 
 BN_lshift() shifts B<a> left by B<n> bits and places the result in