Move i586-unknown-netbsd from tier 2 to tier 3 platform support table

It appears it was intended to be tier 3, but was accidentally added to
tier 2.

Author: Nemo157

src/doc/rustc/src/platform-support.md

@@ -151,7 +151,6 @@ target | std | notes
 `i586-pc-windows-msvc` | * | 32-bit Windows w/o SSE [^x86_32-floats-x87]
 `i586-unknown-linux-gnu` | ✓ | 32-bit Linux w/o SSE (kernel 3.2, glibc 2.17) [^x86_32-floats-x87]
 `i586-unknown-linux-musl` | ✓ | 32-bit Linux w/o SSE, MUSL [^x86_32-floats-x87]
-[`i586-unknown-netbsd`](platform-support/netbsd.md) |  ✓ | 32-bit x86, restricted to Pentium
 [`i686-linux-android`](platform-support/android.md) | ✓ | 32-bit x86 Android [^x86_32-floats-return-ABI]
 `i686-unknown-freebsd` | ✓ | 32-bit FreeBSD [^x86_32-floats-return-ABI]
 `i686-unknown-linux-musl` | ✓ | 32-bit Linux with MUSL [^x86_32-floats-return-ABI]
@@ -270,6 +269,7 @@ target | std | host | notes
 `hexagon-unknown-linux-musl` | ? |  |
 `i386-apple-ios` | ✓ |  | 32-bit x86 iOS [^x86_32-floats-return-ABI]
 [`i586-pc-nto-qnx700`](platform-support/nto-qnx.md) | * |  | 32-bit x86 QNX Neutrino 7.0 RTOS  [^x86_32-floats-return-ABI]
+[`i586-unknown-netbsd`](platform-support/netbsd.md) | ✓ |  | 32-bit x86, restricted to Pentium
 `i686-apple-darwin` | ✓ | ✓ | 32-bit macOS (10.12+, Sierra+) [^x86_32-floats-return-ABI]
 `i686-pc-windows-msvc` | * |  | 32-bit Windows XP support [^x86_32-floats-return-ABI]
 [`i686-pc-windows-gnullvm`](platform-support/pc-windows-gnullvm.md) | ✓ | ✓ | [^x86_32-floats-return-ABI]