Fix cc language ref

Author: bracevac

docs/_docs/reference/experimental/capture-checking/read-only.md renamed to docs/_docs/reference/experimental/capture-checking/mutability.md

@@ -160,6 +160,7 @@ subsection:
           - page: reference/experimental/capture-checking/scoped-caps.md
           - page: reference/experimental/capture-checking/classifiers.md
           - page: reference/experimental/capture-checking/checked-exceptions.md
+          - page: reference/experimental/capture-checking/mutability.md
           - page: reference/experimental/capture-checking/separation-checking.md
           - page: reference/experimental/capture-checking/how-to-use.md
           - page: reference/experimental/capture-checking/internals.md