Switch ABI list to em dashes, fix typographical errors

The ABI list was using em dashes where it should use en dashes and
contained some other typographical errors.  Let's fix these.

Author: traviscross

src/items/external-blocks.md

@@ -111,55 +111,53 @@ r[items.extern.abi.standard]
 The following ABI strings are supported on all platforms:
 
 r[items.extern.abi.rust]
-* `unsafe extern "Rust"` -- The default ABI when you write a normal `fn foo()` in any
+* `unsafe extern "Rust"` --- The default ABI when you write a normal `fn foo()` in any
   Rust code.
 
 r[items.extern.abi.c]
-* `unsafe extern "C"` -- This is the same as `extern fn foo()`; whatever the default
+* `unsafe extern "C"` --- This is the same as `extern fn foo()`; whatever the default
   your C compiler supports.
 
 r[items.extern.abi.system]
-* `unsafe extern "system"` -- Usually the same as `extern "C"`, except on Win32, in
-  which case it's `"stdcall"`, or what you should use to link to the Windows
-  API itself
+* `unsafe extern "system"` --- Usually the same as `extern "C"`, except on Win32, in which case it's `"stdcall"`, or what you should use to link to the Windows API itself.
 
 r[items.extern.abi.unwind]
-* `extern "C-unwind"` and `extern "system-unwind"` -- identical to `"C"` and `"system"`, respectively, but with [different behavior][unwind-behavior] when the callee unwinds (by panicking or throwing a C++ style exception).
+* `extern "C-unwind"` and `extern "system-unwind"` --- Identical to `"C"` and `"system"`, respectively, but with [different behavior][unwind-behavior] when the callee unwinds (by panicking or throwing a C++ style exception).
 
 r[items.extern.abi.platform]
 There are also some platform-specific ABI strings:
 
 r[items.extern.abi.cdecl]
-* `unsafe extern "cdecl"` -- The default for x86_32 C code.
+* `unsafe extern "cdecl"` --- The default for x86_32 C code.
   * Only available on x86_32 targets.
 
 r[items.extern.abi.stdcall]
-* `unsafe extern "stdcall"` -- The default for the Win32 API on x86_32.
+* `unsafe extern "stdcall"` --- The default for the Win32 API on x86_32.
   * Only available on x86_32 targets.
 
 r[items.extern.abi.win64]
-* `unsafe extern "win64"` -- The default for C code on x86_64 Windows.
+* `unsafe extern "win64"` --- The default for C code on x86_64 Windows.
   * Only available on x86_64 targets.
 
 r[items.extern.abi.sysv64]
-* `unsafe extern "sysv64"` -- The default for C code on non-Windows x86_64.
+* `unsafe extern "sysv64"` --- The default for C code on non-Windows x86_64.
   * Only available on x86_64 targets.
 
 r[items.extern.abi.aapcs]
-* `unsafe extern "aapcs"` -- The default for ARM.
+* `unsafe extern "aapcs"` --- The default for ARM.
   * Only available on ARM32 targets.
 
 r[items.extern.abi.fastcall]
-* `unsafe extern "fastcall"` -- The `fastcall` ABI -- corresponds to MSVC's
+* `unsafe extern "fastcall"` --- The `fastcall` ABI --- corresponds to MSVC's
   `__fastcall` and GCC and clang's `__attribute__((fastcall))`.
   * Only available on x86_32 targets.
 
 r[items.extern.abi.thiscall]
-* `unsafe extern "thiscall"` -- The default for C++ member functions on x86_32 MSVC -- corresponds to MSVC's `__thiscall` and GCC and clang's `__attribute__((thiscall))`.
+* `unsafe extern "thiscall"` --- The default for C++ member functions on x86_32 MSVC --- corresponds to MSVC's `__thiscall` and GCC and clang's `__attribute__((thiscall))`.
   * Only available on x86_32 targets.
 
 r[items.extern.abi.efiapi]
-* `unsafe extern "efiapi"` -- The ABI used for [UEFI] functions.
+* `unsafe extern "efiapi"` --- The ABI used for [UEFI] functions.
   * Only available on x86 and ARM targets (32bit and 64bit).
 
 r[items.extern.abi.platform-unwind-variants]