Fix spelling and typographical errors found by cspell

Signed-off-by: Tom Cosgrove <tom.cosgrove@arm.com>
diff --git a/include/mbedtls/chachapoly.h b/include/mbedtls/chachapoly.h
index c4ec7b5..ed568bc 100644
--- a/include/mbedtls/chachapoly.h
+++ b/include/mbedtls/chachapoly.h
@@ -161,7 +161,7 @@
  * \param ctx       The ChaCha20-Poly1305 context. This must be initialized
  *                  and bound to a key.
  * \param nonce     The nonce/IV to use for the message.
- *                  This must be a redable buffer of length \c 12 Bytes.
+ *                  This must be a readable buffer of length \c 12 Bytes.
  * \param mode      The operation to perform: #MBEDTLS_CHACHAPOLY_ENCRYPT or
  *                  #MBEDTLS_CHACHAPOLY_DECRYPT (discouraged, see warning).
  *