Make the name of the "HangupCauseClear" application consistent

The name of the "HangupCauseClear" application is "HangupCauseClear",
not "HangupcauseClear".  The incorrect case of 'cause' caused the
XML documentation to not register properly.

As an aside, this commit message felt very awkward, but I'm not sure
how else to note that "X", which has to be "X", was referred to as "x".

(closes issue ASTERISK-20253)
Reported by: Andrew Latham
Patches:
  hangupcause.diff uploaded by Andrew Latham (license #5985)

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@371516 65c4cc65-6c06-0410-ace0-fbb531ad65f3

funcs/func_hangupcause.c

@@ -218,7 +218,7 @@ static struct ast_custom_function hangupcause_keys_function = {
 	.read = hangupcause_keys_read,
 };
 
-static const char app[] = "HangupcauseClear";
+static const char app[] = "HangupCauseClear";
 
 /*!
  * \internal