Add "ltsc2019" tag to example Windows one-liner (#85)

Author: larryclaman

Makefile

@@ -33,7 +33,7 @@ $(C_TARGETS): hello.c $(MUSL_GCC)
 			'$@' | sed \
 				-e 's/[(]$(TARGET_ARCH)[)]/(windows-$(TARGET_ARCH), '"$$winVariant"')/g' \
 				-e 's/an Ubuntu container/a Windows Server container/g' \
-				-e 's!ubuntu bash!mcr.microsoft.com/windows/servercore powershell!g' \
+				-e 's!ubuntu bash!mcr.microsoft.com/windows/servercore:ltsc2019 powershell!g' \
 				-e 's![$$] docker!PS C:\\> docker!g' \
 				> "$(@D)/$$winVariant/hello.txt"; \
 		done; \

amd64/hello-world/nanoserver-1809/hello.txt

@@ -12,7 +12,7 @@ To generate this message, Docker took the following steps:
     to your terminal.
 
 To try something more ambitious, you can run a Windows Server container with:
- PS C:\> docker run -it mcr.microsoft.com/windows/servercore powershell
+ PS C:\> docker run -it mcr.microsoft.com/windows/servercore:ltsc2019 powershell
 
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