fixup! Base Browser strings

MB 288: Standardize capitalization in Letterboxing preferences

browser/locales/en-US/browser/base-browser.ftl

@@ -84,7 +84,7 @@ letterboxing-window-size-header = Window size
 letterboxing-remember-size =
     .label = Reuse last window size when opening a new window
     .accesskey = R
-letterboxing-alignment-header = Content Alignment
+letterboxing-alignment-header = Content alignment
 letterboxing-alignment-description = Choose where you want to align the website’s content.
 letterboxing-alignment-top = Top
 letterboxing-alignment-middle = Middle