Correct typos that ispell did not find.

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