[i18n] fix translator comments to be visible to gettext

Part of Bug 797270 - Budget Chart report - translation of header and
dates

Previously, the translator comments were on the same line and were not
processed into .po clues -- this commit fixes it.

gnucash/report/standard-reports/budget.scm

@@ -476,15 +476,18 @@
               (when show-budget?
                 (gnc:html-table-set-cell/tag!
                  html-table 1 col0 "centered-label-cell"
-                 (_ "Bgt"))) ;; Translators: Abbreviation for "Budget"
+                 ;; Translators: Abbreviation for "Budget" amount
+                 (_ "Bgt")))
               (when show-actual?
                 (gnc:html-table-set-cell/tag!
                  html-table 1 col1 "centered-label-cell"
-                 (_ "Act"))) ;; Translators: Abbreviation for "Actual"
+                 ;; Translators: Abbreviation for "Actual" amount
+                 (_ "Act")))
               (when show-diff?
                 (gnc:html-table-set-cell/tag!
                  html-table 1 col2 "centered-label-cell"
-                 (_ "Diff"))) ;; Translators: Abbreviation for "Difference"
+                 ;; Translators: Abbreviation for "Difference" amount
+                 (_ "Diff")))
               (loop (cdr column-list)
                     col3))))))