docs: improve language in borrowing section (#2893)

I asked Gemini to review the English for inconsistencies and grammar
mistakes. This is the result and I hope it's useful!

As a non-native speaker, it is hard for me to evaluate the finer
details, so let me know if you would like to see changes (or even
better: make them directly in the PR with the suggestion function).

---------

Co-authored-by: Dmitri Gribenko <gribozavr@gmail.com>

src/borrowing/borrowck.md

@@ -65,9 +65,9 @@ fn main() {
 
 ## More to Explore
 
-- Technically multiple mutable references to a piece of data can exist at the
+- Technically, multiple mutable references to a piece of data can exist at the
   same time via re-borrowing. This is what allows you to pass a mutable
-  reference into a function without invaliding the original reference.
+  reference into a function without invalidating the original reference.
   [This playground example][1] demonstrates that behavior.
 - Rust uses the exclusive reference constraint to ensure that data races do not
   occur in multi-threaded code, since only one thread can have mutable access to