Fix case of Mbed TLS in assemble_changelog.py

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/scripts/assemble_changelog.py b/scripts/assemble_changelog.py
index 7b036aa..f3aca70 100755
--- a/scripts/assemble_changelog.py
+++ b/scripts/assemble_changelog.py
@@ -122,7 +122,7 @@
 class TextChangelogFormat(ChangelogFormat):
     """The traditional Mbed TLS changelog format."""
 
-    _unreleased_version_text = '= mbed TLS x.x.x branch released xxxx-xx-xx'
+    _unreleased_version_text = '= Mbed TLS x.x.x branch released xxxx-xx-xx'
     @classmethod
     def is_released_version(cls, title):
         # Look for an incomplete release date