docs: fix broken antecedent and de-duplicate the cache-button omit note

The omit-cache sentence was spliced mid-paragraph between the raw-id problem
statement and "The cache button fixes that", so "that" resolved to the wrong
antecedent (the omit-cache mechanics instead of the raw-id rendering
problem). The same note already exists standalone later in the section, so
the spliced copy was also a duplicate. Drop it from the lead paragraph and
keep the one standalone copy.

Signed-off-by: disqualifier <dev@disqualifier.me>

README.md

@@ -115,8 +115,6 @@ echoes its `data`.
 ## Cache button (mention priming)
 
 Discord clients render `<@id>` as a raw id until the user object is cached locally.
-Omit `cache` (or pass `None` / `[]`) and the button never appears; a non-empty cache
-must have one entry per page or construction raises.
 The cache button fixes that: pass `cache=[...]` with one entry per page, where each
 entry is a string of the user mentions on that page (`"<@111> <@222> <@333>"`).