Fix dead link

Author: Blaisorblade

docs/docs/reference/principled-meta-programming.md

@@ -751,7 +751,7 @@ environments and terms.
                            Es |- 't: expr T
 
 The meta theory of a slightly simplified variant 2-stage variant of this calculus
-is studied [separatey](../simple-smp.md)
+is studied [separately](simple-smp.md).
 
 ## Going Further