[spec] Update limits type in wasm-3.0 text format spec

[keithw]

In memory64, the types of limits min and max were updated to u64 in the abstract syntax and the binary format specification. This updates the text format spec to match.

This is already tested by the wasm-3.0 tests, e.g. this assert_invalid test from memory.wast is an assert_malformed on the main branch:

(assert_invalid
  (module (memory 0x1_0000_0000))
  "memory size"
)

[rossberg]

Thanks!