Fix typos in C header files

Signed-off-by: Tobias Nießen <tniessen@tnie.de>
diff --git a/include/mbedtls/ecp.h b/include/mbedtls/ecp.h
index 149bda0..dd400a0 100644
--- a/include/mbedtls/ecp.h
+++ b/include/mbedtls/ecp.h
@@ -1176,7 +1176,7 @@
  *
  * \param grp_id    The ECP group identifier.
  * \param key       The destination key.
- * \param buf       The the buffer containing the binary representation of the
+ * \param buf       The buffer containing the binary representation of the
  *                  key. (Big endian integer for Weierstrass curves, byte
  *                  string for Montgomery curves.)
  * \param buflen    The length of the buffer in bytes.