modelfile*.txt: use POSIX path separator (/)

Use the POSIX file separator (a slash, /)
instead of the Windows one (a backslash, \)
to support Unix(-like) platforms like MacOS and Linux.

The ollama documentation on modelfiles uses the POSIX path separator[1],
which might indicate their preference on such representation.

[1]: https://github.com/ollama/ollama/blob/main/docs/modelfile.md#build-from-a-gguf-file

modelfilegemmapro-20kctx.txt

@@ -1,4 +1,4 @@
-FROM .\GemmaPro_q4.gguf
+FROM ./GemmaPro_q4.gguf
 
 PARAMETER num_ctx 20000
 

modelfilegemmapro-r.txt

@@ -1,4 +1,4 @@
-FROM .\GemmaPro_q4.gguf
+FROM ./GemmaPro_q4.gguf
 
 PARAMETER num_ctx 2048
 

modelfilegemmapro.txt

@@ -1,4 +1,4 @@
-FROM .\GemmaPro_q4.gguf
+FROM ./GemmaPro_q4.gguf
 
 PARAMETER num_ctx 2048