I think it would be more like this, but I haven't tested the code:
import os
from glob import glob
import arcpy
PATH = r'C:\Users\mclbr\Desktop\GIS_Maintenance'
arcpy.env.workspace = PATH
result = [y for x in os.walk(PATH) for y in glob(os.path.join(x[0], '*.mxd'))]
for map in result:
print "Processing: {}".format(map)
mxd = arcpy.mapping.MapDocument(map)
mxd.relativePaths = True
mxd.save()
You can add some additional print statements to help keep track of what's happening and to assist with debugging.
I'm not sure how the arcpy workspace plays into relative paths. You may need to move this into your for loop and set workspace equal to the path up to your mxd file.