Fix spelling in `doc/mruby3.2.md`
* Now `a::B = c` should evaluate `a` then `c`.
* Anonymous arguments `*`, `**`, `&` can be passed for forwarding.
-* Multi-precition integer is available now via `mruby-bigint` gem.
+* Multi-precision integer is available now via `mruby-bigint` gem.
# Tools
