Fix incorrect parameter name in mbedtls_mpi_core_add_if() doc comment

Signed-off-by: Tom Cosgrove <tom.cosgrove@arm.com>
diff --git a/library/bignum_core.h b/library/bignum_core.h
index 7fd6fed..e443d2c 100644
--- a/library/bignum_core.h
+++ b/library/bignum_core.h
@@ -256,7 +256,7 @@
  * \param cond       Condition bit dictating whether addition should
  *                   happen or not. This must be \c 0 or \c 1.
  *
- * \warning          If \p assign is neither 0 nor 1, the result of this function
+ * \warning          If \p cond is neither 0 nor 1, the result of this function
  *                   is unspecified, and the resulting value in \p A might be
  *                   neither its original value nor \p A + \p B.
  *