mnemonics: comma means no input

TODO: unit test

Author: vadosnaprimer

src/BizHawk.Emulation.Common/Base Implementations/Bk2MnemonicLookup.cs

@@ -702,7 +702,7 @@ public static string LookupAxis(string button, string systemId)
 				["Apostrophe"] = '\'',
 				["Grave"] = '`',
 				["Comma"] = ',',
-				["Period"] = '.',
+				["Period"] = 'p',
 				["Slash"] = '/',
 				["Space"] = 's',
 				["Tab"] = 't',