nit: fixed capitalization of dark theme

Author: aritroCoder

src/emoji/codePointMap.js

@@ -20,7 +20,7 @@ export const override: {| [code: string]: string |} = {
 
   // Fix the "letter" emoji.  The codes in Zulip's emoji database would give
   // these a text-style rather than emoji-style presentation; that's subtler
-  // than we want, plus when not in Dark theme it's actually invisible.
+  // than we want, plus when not in dark theme it's actually invisible.
   '1f170': '1f170-fe0f', // :a:
   '1f171': '1f171-fe0f', // :b:
   '1f17e': '1f17e-fe0f', // :o: