fix: C API Datalog example — result columns are reach__c0, reach__c1

Author: singaraiona

website/docs/c-api-dag.html

@@ -770,7 +770,7 @@ <h4>Datalog example: transitive closure</h4>
     <span class="hl-fn">dl_stratify</span>(prog);
     <span class="hl-fn">dl_eval</span>(prog);
     <span class="hl-type">ray_t</span>* result = <span class="hl-fn">dl_query</span>(prog, <span class="hl-str">"reach"</span>);
-    <span class="hl-comment">/* result is a table with columns c0, c1:
+    <span class="hl-comment">/* result is a table with columns reach__c0, reach__c1:
      *   0 | 1
      *   1 | 2
      *   2 | 3