Fix doumentation

Signed-off-by: Gabor Mezei <gabor.mezei@arm.com>
diff --git a/library/bignum_core.h b/library/bignum_core.h
index cb93238..d895148 100644
--- a/library/bignum_core.h
+++ b/library/bignum_core.h
@@ -101,7 +101,7 @@
                                    unsigned char assign );
 
 /**
- * \brief   Perform a safe conditional copy of MPI which doesn't reveal whether
+ * \brief   Perform a safe conditional swap of MPI which doesn't reveal whether
  *          the condition was true or not.
  *
  * \param[IN,OUT] X         The address of the first MPI.