change main dilution input

sample_prep_protocol.html

@@ -78,12 +78,12 @@
         <div class="row">
         	<div class="col-6">
             <label class = "form-label" for="main_dilution">
-              Main dilution
+              Main dilution factor (1:x)
             </label>
           </div>
           <div class="col">
             <input class="form-control" type="text" id = "Main_Dilution" name="main_dilution"
-                     placeholder="e.g. 1:5">
+                     placeholder="e.g. 5 for a 1:5 dilution">
             </input>
       	  </div>
       	</div>