Fix typos in doxygen commands in new bignum modules

Signed-off-by: Tom Cosgrove <tom.cosgrove@arm.com>
diff --git a/library/bignum_mod.h b/library/bignum_mod.h
index d8c8b7d..d4c1d5d 100644
--- a/library/bignum_mod.h
+++ b/library/bignum_mod.h
@@ -444,7 +444,7 @@
  *                  limbs as the modulus \p N. (\p r is an input parameter, but
  *                  its value will be modified during execution and restored
  *                  before the function returns.)
- * \param[in] N     The address of the modulus associated with \r.
+ * \param[in] N     The address of the modulus associated with \p r.
  * \param[out] buf  The output buffer to export to.
  * \param buflen    The length in bytes of \p buf.
  * \param ext_rep   The endianness in which the number should be written into