Update spelling "mbed TLS" to "Mbed TLS"

The official spelling of the trade mark changed from all-lowercase "mbed"
to normal proper noun capitalization "Mbed" a few years ago. We've been
using the new spelling in new text but still have the old spelling in a
lot of text. This commit updates most occurrences of "mbed TLS":

```
sed -i -e 's/mbed TLS/Mbed TLS/g' $(git ls-files ':!ChangeLog' ':!tests/data_files/**' ':!tests/suites/*.data' ':!programs/x509/*' ':!configs/tfm*')
```

Justification for the omissions:

* `ChangeLog`: historical text.
* `test/data_files/**`, `tests/suites/*.data`, `programs/x509/*`: many
  occurrences are significant names in certificates and such. Changing
  the spelling would invalidate many signatures and tests.
* `configs/tfm*`: this is an imported file. We'll follow the upstream
  updates.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/include/mbedtls/version.h b/include/mbedtls/version.h
index bb1a4c3..073211a 100644
--- a/include/mbedtls/version.h
+++ b/include/mbedtls/version.h
@@ -52,9 +52,9 @@
 void mbedtls_version_get_string(char *string);
 
 /**
- * Get the full version string ("mbed TLS x.y.z").
+ * Get the full version string ("Mbed TLS x.y.z").
  *
- * \param string    The string that will receive the value. The mbed TLS version
+ * \param string    The string that will receive the value. The Mbed TLS version
  *                  string will use 18 bytes AT MOST including a terminating
  *                  null byte.
  *                  (So the buffer should be at least 18 bytes to receive this
@@ -64,12 +64,12 @@
 
 /**
  * \brief           Check if support for a feature was compiled into this
- *                  mbed TLS binary. This allows you to see at runtime if the
+ *                  Mbed TLS binary. This allows you to see at runtime if the
  *                  library was for instance compiled with or without
  *                  Multi-threading support.
  *
  * \note            only checks against defines in the sections "System
- *                  support", "mbed TLS modules" and "mbed TLS feature
+ *                  support", "Mbed TLS modules" and "Mbed TLS feature
  *                  support" in mbedtls_config.h
  *
  * \param feature   The string for the define to check (e.g. "MBEDTLS_AES_C")