Fix bug that occurs when opening help documents

[ravibrock]

Escape tildes when calling fn.matchadd("CursorWord", pattern, -1) so that it doesn't throw the error Vim:E874: (NFA) Could not pop the stack! when scrolling through help documents like the following (:h cmp_docs):

Config                                                              *cmp-config*

You can use the following options via `cmp.setup { ... }` .

                                                            *cmp-config.enabled*
enabled~ <-- TILDE HERE CAUSES ERROR
  `boolean | fun(): boolean`
  Toggles the plugin on and off.

[sadtab]

I manually patched it, thanks