use md extension instead of html

Author: ehuss

src/expressions/underscore-expr.md

@@ -8,7 +8,7 @@ Underscore expressions, denoted with the symbol `_`, are used to signify a
 placeholder in a destructuring assignment. They may only appear in the left-hand
 side of an assignment.
 
-Note that this is distinct from the [wildcard pattern](../patterns.html#wildcard-pattern).
+Note that this is distinct from the [wildcard pattern](../patterns.md#wildcard-pattern).
 
 An example of an `_` expression: