Update vmlDrawing in worksheets_rels

[JanMarvin]

There is an unrelated bug in charts vml handling (in this case it is related to embedded files, therefore most likely not that visible to users):

library(openxlsx2)

fl <- system.file("extdata", "mtcars_chart.xlsx", package = "openxlsx2")
wb <- wb_load(fl)
wb$save("/tmp/test.xlsx")

wb$worksheets_rels[[4]][2] <- # vmlDrawing is now vmlDrawing4 not 1
  "<Relationship Id=\"rId2\" Type=\"http://schemas.openxmlformats.org/officeDocument/2006/relationships/vmlDrawing\" Target=\"../drawings/vmlDrawing4.vml\"/>" 
  
wb$open()

unlink("/tmp/xlsx", recursive = TRUE)
unlink("/tmp/test", recursive = TRUE)

unzip(fl, exdir = "/tmp/xlsx")
unzip("/tmp/test.xlsx", exdir = "/tmp/test")