Fix typos in doxygen commands in new bignum modules

Signed-off-by: Tom Cosgrove <tom.cosgrove@arm.com>
diff --git a/library/bignum_core.h b/library/bignum_core.h
index 44d5c5a..4fb8f65 100644
--- a/library/bignum_core.h
+++ b/library/bignum_core.h
@@ -222,7 +222,7 @@
  * most significant zero bytes in the input).
  *
  * \param[out] X        The address of the MPI.
- *                      May only be #NULL if \X_limbs is 0 and \p input_length
+ *                      May only be #NULL if \p X_limbs is 0 and \p input_length
  *                      is 0.
  * \param X_limbs       The number of limbs of \p X.
  * \param[in] input     The input buffer to import from.