Minor grammar fix in comment

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
diff --git a/library/rsa_alt_helpers.h b/library/rsa_alt_helpers.h
index 580ffb2..8ff74a0 100644
--- a/library/rsa_alt_helpers.h
+++ b/library/rsa_alt_helpers.h
@@ -90,7 +90,7 @@
  * \param Q        Second prime factor of RSA modulus
  * \param E        RSA public exponent
  * \param D        Pointer to MPI holding the private exponent on success,
- *                 ie the modular inverse of E modulo LCM(P-1,Q-1).
+ *                 i.e. the modular inverse of E modulo LCM(P-1,Q-1).
  *
  * \return         \c 0 if successful.
  * \return         #MBEDTLS_ERR_MPI_ALLOC_FAILED if a memory allocation failed.