Update sha256.h

Minor fix based on review comments
diff --git a/include/mbedtls/sha256.h b/include/mbedtls/sha256.h
index 7affb1b..c4465e5 100644
--- a/include/mbedtls/sha256.h
+++ b/include/mbedtls/sha256.h
@@ -102,7 +102,7 @@
  * \brief          This function feeds an input buffer into an ongoing
  *                 SHA-256 checksum calculation.
  *
- * \param ctx      The SHA-256 context to initialize.
+ * \param ctx      The SHA-256 context.
  * \param input    The buffer holding the data.
  * \param ilen     The length of the input data.
  *