Change ABI string from sysV64 to sysv64

Author: CensoredUsername

src/doc/book/ffi.md

@@ -539,7 +539,7 @@ This is currently hidden behind the `abi_vectorcall` gate and is subject to chan
 * `system`
 * `C`
 * `win64`
-* `sysV64`
+* `sysv64`
 
 Most of the abis in this list are self-explanatory, but the `system` abi may
 seem a little odd. This constraint selects whatever the appropriate ABI is for

src/libsyntax/abi.rs

@@ -87,7 +87,7 @@ const AbiDatas: &'static [AbiData] = &[
     AbiData {abi: Abi::Vectorcall, name: "vectorcall"},
     AbiData {abi: Abi::Aapcs, name: "aapcs" },
     AbiData {abi: Abi::Win64, name: "win64" },
-    AbiData {abi: Abi::SysV64, name: "sysV64" },
+    AbiData {abi: Abi::SysV64, name: "sysv64" },
 
     // Cross-platform ABIs
     //