Fix missing "for" attribute in officer assignment label

Since this attribute was missing, the label wasn't correctly associated
with its field.

app/views/officing/results/new.html.erb

@@ -4,7 +4,7 @@
   <%= form_tag(officing_poll_results_path(@poll), { id: "officer_assignment_form" }) do %>
     <div class="row">
       <div class="small-12 medium-6 column">
-        <label><%= t("officing.results.new.booth") %></label>
+        <%= label_tag :officer_assignment_id, t("officing.results.new.booth") %>
         <%= select_tag :officer_assignment_id,
                        booths_for_officer_select_options(@officer_assignments),
                        { prompt: t("officing.results.new.select_booth") } %>

spec/system/budget_polls/ballot_sheets_spec.rb

@@ -103,7 +103,7 @@ describe "Poll budget ballot sheets" do
     scenario "Ballot sheet is saved" do
       visit new_officing_poll_ballot_sheet_path(poll)
 
-      select booth.name, from: "officer_assignment_id"
+      select booth.name, from: "Booth"
       fill_in "data", with: "1234;5678"
       click_button "Save"
 
@@ -119,7 +119,7 @@ describe "Poll budget ballot sheets" do
     scenario "Ballot sheet is not saved" do
       visit new_officing_poll_ballot_sheet_path(poll)
 
-      select booth.name, from: "officer_assignment_id"
+      select booth.name, from: "Booth"
       click_button "Save"
 
       expect(page).to have_content("CSV data can't be blank")

spec/system/officing/results_spec.rb

@@ -57,7 +57,7 @@ describe "Officing Results", :with_frozen_time do
 
     expect(page).not_to have_content("Your results")
 
-    select booth.name, from: "officer_assignment_id"
+    select booth.name, from: "Booth"
 
     fill_in "questions[#{question_1.id}][0]", with: "100"
     fill_in "questions[#{question_1.id}][1]", with: "200"
@@ -100,7 +100,7 @@ describe "Officing Results", :with_frozen_time do
     visit new_officing_poll_result_path(poll)
 
     booth_name = partial_result.booth_assignment.booth.name
-    select booth_name, from: "officer_assignment_id"
+    select booth_name, from: "Booth"
 
     fill_in "questions[#{question_1.id}][0]", with: "5555"
     fill_in "questions[#{question_1.id}][1]", with: "200"