CPP: Fix type 'diagnosstic'.

Author: geoffw0

cpp/ql/src/semmlecode.cpp.dbscheme

@@ -98,7 +98,7 @@ compilation_time(
 
 /**
  * An error or warning generated by the extractor.
- * The diagnosstic message `diagnostic` was generated during compiler
+ * The diagnostic message `diagnostic` was generated during compiler
  * invocation `compilation`, and is the `file_number_diagnostic_number`th
  * message generated while extracting the `file_number`th file of that
  * invocation.