[PATCH] Change slash to backslash in make.bat

Hi, all

In windows, backslash should be used in the path. 
In Linux, slash should be used in path.
But in pgadmin4\make.bat, slash is used in the path.

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FOR /f "tokens=1 DELims=." %%G IN ('%PGADMIN_PYTHON_DIR%/python.exe -c "print('%APP_NAME%'.lower().replace(' ', ''))"')
DOSET APP_SHORTNAME=%%G
 

FOR /f "tokens=1 DELims=." %%G IN ('%PGADMIN_PYTHON_DIR%/python.exe -c "import sys; print(sys.version.split(' ')[0])"')
DOSET PYTHON_MAJOR=%%G
 
FOR /f "tokens=2 DELims=." %%G IN ('%PGADMIN_PYTHON_DIR%/python.exe -c "import sys; print(sys.version.split(' ')[0])"')
DOSET PYTHON_MINOR=%%G
 
FOR /f "tokens=3 DELims=." %%G IN ('%PGADMIN_PYTHON_DIR%/python.exe -c "import sys; print(sys.version.split(' ')[0])"')
DOSET PYTHON_REVISION=%%G
 

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Although the slash does not affect the compilation, I think it is better to modify it.

Here is a patch for make.bat.
Please review.

Best Regards!
Zhang Jie