md5(1): fix style in man page

Fix a bunch of style problems reported by mandoc(1) and igor:

mandoc: ./md5.1:19:71: STYLE: no blank before trailing delimiter: Nm ... rmd160,
mandoc: ./md5.1:20:23: STYLE: no blank before trailing delimiter: Nm ... skein512,
mandoc: ./md5.1:33:2: STYLE: useless macro: Tn
mandoc: ./md5.1:33:2: STYLE: useless macro: Tn
mandoc: ./md5.1:33:2: STYLE: useless macro: Tn
mandoc: ./md5.1:33:2: STYLE: useless macro: Tn
mandoc: ./md5.1:35:2: STYLE: useless macro: Tn
mandoc: ./md5.1:42:2: STYLE: useless macro: Tn
mandoc: ./md5.1:45:2: STYLE: useless macro: Tn
mandoc: ./md5.1:47:2: STYLE: useless macro: Tn
mandoc: ./md5.1:56:2: STYLE: useless macro: Tn
mandoc: ./md5.1:58:2: STYLE: useless macro: Tn
mandoc: ./md5.1:61:2: STYLE: useless macro: Tn
mandoc: ./md5.1:66:2: STYLE: useless macro: Tn
mandoc: ./md5.1:68:2: STYLE: useless macro: Tn
mandoc: ./md5.1:104:24: STYLE: no blank before trailing delimiter: Nm skein512,
mandoc: ./md5.1:117:6: STYLE: referenced manual not found: Xr sha224 3

igor:
md5.1:46:no comma after "i.e.":either algorithm, [i.e.] to find an input that produces a specific

Approved by: bcr@
Differential Revision: https://reviews.freebsd.org/D25277