Replace 'thread safe' by 'thread-safe' in the documentation

ChangeLog

@@ -4,7 +4,7 @@ mbed TLS ChangeLog (Sorted per branch, date)
 
 API Changes
    * Extend the platform module with an abstraction mbedtls_platform_gmtime_r()
-     whose implementation should behave as a thread safe version of gmtime().
+     whose implementation should behave as a thread-safe version of gmtime().
      This allows users to configure such an implementation at compile time when
      the target system cannot be deduced automatically, by setting the option
      MBEDTLS_PLATFORM_GMTIME_R_ALT. At this stage Mbed TLS is only able to

include/mbedtls/config.h

@@ -147,7 +147,7 @@
  *
  * \note mbedtls_platform_gmtime_r() is an abstraction in platform_util.h that
  * behaves similarly to the gmtime() function from the C standard,
- * but is thread safe. Mbed TLS will try to identify the underlying platform
+ * but is thread-safe. Mbed TLS will try to identify the underlying platform
  * and configure an appropriate underlying implementation (e.g. gmtime_r() for
  * POSIX and gmtime_s() for Windows). If this is not possible, then
  * gmtime() will be used. Refer to the documentation for
@@ -3101,7 +3101,7 @@
  * mbedtls_platform_gmtime_r(). This replaces the default implementation in
  * platform_util.c.
  *
- * gmtime() is not a thread safe function as defined in the C standard. The
+ * gmtime() is not a thread-safe function as defined in the C standard. The
  * library will try to use safer implementations of this function, such as
  * gmtime_r() when available. However, if Mbed TLS cannot identify the target
  * system, the implementation of mbedtls_platform_gmtime_r() will default to

include/mbedtls/platform_util.h

@@ -68,7 +68,7 @@ void mbedtls_platform_zeroize( void *buf, size_t len );
 
 #if defined(MBEDTLS_HAVE_TIME_DATE)
 /**
- * \brief      Thread safe implementation of gmtime()
+ * \brief      Thread-safe implementation of gmtime()
  *
  *             The function is an abstraction that when called behaves similar
  *             to the gmtime() function from the C standard, but is thread