std.mem.readVarInt: Fix type name in doc comment (#31007)

Code used `ReturnType`, comment used `T` (which is what is used in similar functions).

Reviewed-on: https://codeberg.org/ziglang/zig/pulls/31007
Co-authored-by: Robert Ancell <robert.ancell@gmail.com>
Co-committed-by: Robert Ancell <robert.ancell@gmail.com>
0af79e7b8cRobert Ancell committed on 1/27/2026, 4:29:51 AM· committed by Alex Rønne Petersenparent951ab1b
1 file changedLine totals unavailable