Fixed typo

notes/arch.md

     [ STM ]
 
        | 1      parse_head()    # parse text and fill out metadata properties
-       |        gen_sts(args)   # generate a number of STS from a STM
+       |        gen_sts(args)   # generate a number of STS from an STM
        |                        # * args - args specify ranges from where
        |                        #          STMs are generated; typically
        |                        #          Cartesian product