QtHelp: Update Sphinx issue URLs in comments

The QtHelp Sphinx generation code contains two links to Sphinx issues
that used old, dead BitBucket URLs. Those issues were migrated to
GitHub, so the dead links can be replaced with working ones.

Utilities/Sphinx/CMakeLists.txt

@@ -118,12 +118,12 @@ if(SPHINX_QTHELP)
     COMMAND ${CMAKE_COMMAND} "-DCSS_DIR=${CMAKE_CURRENT_BINARY_DIR}/qthelp/_static"
       -P "${CMAKE_CURRENT_SOURCE_DIR}/apply_qthelp_css_workaround.cmake"
     # Workaround sphinx configurability:
-    # https://bitbucket.org/birkenfeld/sphinx/issue/1448/make-qthelp-more-configurable
+    # https://github.com/sphinx-doc/sphinx/issues/1448
     COMMAND ${CMAKE_COMMAND} "-DQTHELP_DIR=${CMAKE_CURRENT_BINARY_DIR}/qthelp/"
       -P "${CMAKE_CURRENT_SOURCE_DIR}/fixup_qthelp_names.cmake"
 
     # Create proper identifiers. Workaround for
-    # https://bitbucket.org/birkenfeld/sphinx/issue/1491/qthelp-should-generate-identifiers-for
+    # https://github.com/sphinx-doc/sphinx/issues/1491
     COMMAND "${Python_EXECUTABLE}"
       "${CMAKE_CURRENT_SOURCE_DIR}/create_identifiers.py"
       "${CMAKE_CURRENT_BINARY_DIR}/qthelp/"