Fix autocucumber in documentation

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

Author: gilles-peskine-arm

tests/include/test/helpers.h

@@ -366,7 +366,7 @@ void mbedtls_test_err_add_check( int high, int low,
  * - This function guarantees that if \p s begins with '-' then the sign
  *   bit of the result will be negative, even if the value is 0.
  *   When this function encounters such a "negative 0", it
- *   increments #mbedtls_test_read_mpi.
+ *   increments #mbedtls_test_case_uses_negative_0.
  * - The size of the result is exactly the minimum number of limbs needed
  *   to fit the digits in the input. In particular, this function constructs
  *   a bignum with 0 limbs for an empty string, and a bignum with leading 0