Update arm64ec-windows-abi-conventions.md

engineer review

Author: TylerMSFT

docs/build/arm64ec-windows-abi-conventions.md

@@ -50,7 +50,7 @@ Special helper routines like `__chkstk_arm64ec` use custom calling conventions a
 | `x15` | `mm7` (low 64 bits of x87 `R7` register) | volatile | volatile | volatile |
 | `x16` | High 16 bits of each of the x87 `R0`-`R3` registers | volatile(`xip0`) | volatile(`xip0`) | volatile |
 | `x17` | High 16 bits of each of the x87 `R4`-`R7` registers | volatile(`xip1`) | volatile(`xip1`) | volatile |
-| `x18` | GS.base | fixed(TEB) | fixed(TEB) | N/A |
+| `x18` | GS.base | fixed(TEB) | fixed(TEB) | fixed(TEB) |
 | `x19` | `r12` | non-volatile | non-volatile | non-volatile |
 | `x20` | `r13` | non-volatile | non-volatile | non-volatile |
 | `x21` | `r14` | non-volatile | non-volatile | non-volatile |