fix: correct description for gap

`gap=1px` means 1px around each module,
i.e. 2px between modules, not 1px.
yep, i know, such a bad name for this parameter,
but who cares?

index.html

@@ -34,7 +34,7 @@
       <label for="gap">
         <span>Grid gap:</span>
         <input id="gap" type="number" min="0" value="0">
-        px between modules
+        px around
       </label>
       <button id="gen-btn">Generate</button>
     </div>